JP4047599B2 - D / A converter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a D / A converter for converting a digital signal into an analog signal.
[0002]
[Prior art]
In recent years, due to improvements in semiconductor integration technology, the system has been integrated on a single chip. Therefore, when an analog circuit is built in a semiconductor device, the performance is improved while suppressing an increase in the circuit area and the number of terminals. The challenge is how to plan.
[0003]
Among commonly used analog circuit D / A converter systems, there is an R-2R ladder resistor type D / A converter shown in FIG. 10 (for example, a 4-bit configuration). This D / A converter uses resistors with resistance values R and 2R, controls the switch according to the value of each bit, and divides the voltage according to whether the resistor is connected to the reference voltage VREFH or VREFL. An analog signal having a dynamic range of (VREFH−VREFL) × (2 ⁿ −1) / 2 ⁿ + VREFL is output. Due to such a simple configuration, a small-area D / A converter circuit can be realized relatively easily.
[0004]
[Problems to be solved by the invention]
However, in the configuration of the conventional R-2R ladder resistor D / A converter, the output of the R-2R resistor network is an analog output as it is, so that the output impedance is high. It is easily affected by the load connected to the output, such as a low-pass filter for smoothing the waveform. For example, in the case of a signal that changes at a high speed, there is a problem that the response of the analog output is lowered due to the rounding of the waveform as shown in FIG.
[0005]
The present invention has been made to solve the above problems, and an object of the present invention is to provide a D / A conversion device that improves the response to a signal that changes at high speed by improving output impedance characteristics. And
[0006]
[Means for Solving the Problems]
In order to achieve the above object, a D / A converter according to the present invention converts a digital signal into an analog signal by using a first reference voltage and a second reference voltage lower than the first reference voltage. R-2R ladder resistance type D / A conversion means, and impedance conversion means for impedance conversion of an analog signal received from the D / A conversion means, the second reference voltage is a ground voltage, and the R- The 2R ladder resistance type D / A conversion means includes a plurality of first resistors (each resistance value is R) connected in series from the output of the R-2R ladder resistance type D / A conversion means, and the first resistance And a plurality of second resistors (each resistance value is 2R) respectively connected to both end points of the resistor group including the plurality of first resistors connected in series, and the second resistors Connected to each end point that is not on the first resistance side. The second resistor, the first reference voltage or a plurality of switches for connecting the ground voltage, one end connected to a first reference voltage in common with the plurality of switches, the other end said R-2R ladder, The plurality of switches are controlled based on a third resistor (resistance value is 2R) connected to the output of the resistance type D / A converter and different from the second resistor, and the input digital signal. It possesses a switch control means, wherein the impedance converting means includes a N-type MOS transistor the analog signal from the R-2R ladder resistor-type D / a converter is input to the gate, connected to the N-type MOS transistor It is characterized by having a resistance .
[0007]
The D / A converter according to the present invention is characterized in that, in the D / A converter, the R-2R ladder resistor type D / A converter gives an offset voltage to the analog signal. .
[0011]
The D / A converter according to the present invention is characterized in that, in the D / A converter, the impedance converter has a P-type MOS transistor instead of the N-type MOS transistor.
[0012]
The D / A converter according to the present invention is characterized in that, in the D / A converter, the impedance converter has an N-type MOS transistor or a P-type MOS transistor instead of the resistor. is there.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
(Embodiment 1)
Hereinafter, a D / A converter according to Embodiment 1 of the present invention will be described with reference to the drawings. The D / A converter according to the first embodiment improves the current output capability of the analog output of the R-2R ladder resistor type D / A converter and improves the response of the analog output signal.
[0014]
FIG. 1 is a circuit diagram showing the configuration of the D / A converter according to the first embodiment, and FIG. 2 is a waveform diagram for explaining the operational effects of the D / A converter.
In FIG. 1, the D / A converter 100 according to the first embodiment includes an R-2R ladder resistance type D / A converter 101 and an impedance converter 102. The R-2R ladder resistance type D / A converter 101 converts a digital signal into an analog signal using a reference voltage VREFH and a reference voltage VREFL (where VREFL is lower than VREFH), and , A switch control 1010, switches 1011a, 1011b, 1011c, and 1011d, and a plurality of resistors having a resistance value of 2R or R.
[0015]
The switch control 1010 controls the switch 1011a and the like based on the input digital signal. When each bit of the input digital signal is “1”, the switch selects the reference voltage VREFH side, and when it is “0”. The switch selects the reference voltage VREFL side. For example, when the LSB is “0”, the switch 1011a is made to select the VREFL side, and when the LSB is “1”, the switch 1011a is made to select the VREFH side. By controlling the switch 1011a and the like by the switch control 1010 in this way, the voltage from the reference voltage VREFL to the reference voltage VREFH is divided, and from VREFL to (VREFH−VREFL) × (2 ⁿ −1) / 2 ⁿ An analog signal having a dynamic range of + VREFL is output from the R-2R ladder resistance type D / A conversion means 101.
[0016]
The impedance conversion means 102 converts the analog signal received from the R-2R ladder resistance type D / A conversion means 101, and is connected to the N-type MOS transistor 104 and the source of the N-type MOS transistor 104. And a resistor 103. This source output (VOUT) is the final analog output of the D / A converter according to the first embodiment.
[0017]
Next, the operation of the D / A converter according to the first embodiment will be described.
The digital signal is input to the switch control 1010, and the switches 1011a to 1011d are controlled according to the value of each bit of the digital signal. The voltage from the reference voltage VREFL to the reference voltage VREFH is divided by the control of the switches 1011a to 1011d. Then, an analog signal that changes every VREFL by (VREFH−VREFL) / 2 ⁿ is output from the R-2R ladder resistance type D / A conversion means 101.
[0018]
The output from the R-2R ladder resistance type D / A conversion means 101 is input to the gate of the N-type MOS transistor 104, and the impedance is converted by the impedance conversion means 102. Therefore, although the output impedance of the R-2R ladder resistance type D / A conversion unit 101 is high, the output from the impedance conversion unit 102, that is, the output (VOUT) from the D / A conversion device 100 has improved current capability. The impedance is low. As shown in FIG. 2, even when the output (VOUT) of the D / A converter 100 is compared with the output (AOUT) after passing the output through the low-pass filter 105, the conventional example (FIG. 11). As can be seen from FIG.
[0019]
As described above, according to the D / A conversion device of the first embodiment, the impedance conversion unit 102 of the source follower circuit including the N-type MOS transistor 104 and the resistor 103 is provided, and the impedance conversion unit 102 allows the R / R conversion unit 102 to -2R ladder resistance type D / A converter 101 converts the impedance of the analog output to improve the output impedance characteristics of the D / A converter, improve the output current drive capability, and change at high speed Responsiveness to signals can be improved.
[0020]
(Embodiment 2)
Hereinafter, a D / A converter according to Embodiment 2 of the present invention will be described with reference to the drawings.
FIG. 3 is a circuit diagram showing a configuration of the D / A converter according to the first embodiment described above, in which the reference voltage VREFL is used as the ground voltage, and FIG. 4 shows the impedance converter in the D / A converter. It is a wave form diagram which shows the input waveform and output waveform of. Note that the configuration and operation of the D / A converter shown in FIG. 3 are the same as those in the first embodiment, and a description thereof will be omitted.
[0021]
As shown in FIG. 3, the reference voltages VREFH and VREFL of the R-2R ladder resistance type D / A conversion means 101 are the reference voltage VREFH and the power supply voltage VDD of the semiconductor device incorporating the D / A conversion device. In general, the reference voltage VREFL is set to the ground voltage (ground). In this case, as described above, the signal response can be improved by the impedance conversion means 102 by the source follower. However, in order for the N-type MOS transistor 104 of the impedance conversion means 102 to operate, the R-2R ladder is used. The voltage of the analog signal output S11 of the resistance type D / A conversion unit 101 needs to be equal to or higher than the threshold voltage VT of the N-type MOS transistor 104. For example, as shown in FIG. 4A, when the analog signal output S11 is a signal having the full dynamic range of the R-2R ladder resistance type D / A conversion unit 101, the output from the impedance conversion unit 102 That is, the output S12 from the D / A converter is as shown in FIG. 4B, and signal loss occurs by the threshold voltage VT of the N-type MOS transistor 104. The D / A converter according to the second embodiment improves the lack of analog output signal.
[0022]
FIG. 5 is a circuit diagram showing a configuration of the D / A converter according to the second embodiment, and FIG. 6 is a waveform diagram showing an input waveform and an output waveform to impedance conversion means in the D / A converter. is there.
[0023]
5, in the D / A converter according to the second embodiment, as the reference voltages VREFH and VREFL used in the R-2R ladder resistance type D / A converter 101, the reference voltage VREFH is the power supply voltage VDD1, and the low-voltage side The reference voltage VREFL is set to another power supply voltage VDD2. Here, the voltage VDD2 is equal to or higher than the threshold voltage VT of the N-type MOS transistor 104 and is lower than the power supply voltage VDD1. Other configurations and operations of the D / A conversion apparatus are the same as those in the first embodiment, and a description thereof will be omitted.
[0024]
In the D / A converter according to the second embodiment, the offset voltage is given to the analog output of the R-2R ladder resistor type D / A converter 101 by setting the reference voltage VREFL to VDD2, which is not the ground voltage. As a result, the analog signal output S21 of the R-2R ladder resistance type D / A conversion means 101 is as shown in FIG. Therefore, since the signal input to the gate of the N-type MOS transistor 104 is equal to or higher than the threshold voltage VT, the output of the impedance conversion means 102 is as shown in FIG.
[0025]
As described above, according to the D / A converter according to the second embodiment, the offset voltage is applied to the analog output of the R-2R ladder resistance type D / A converter 101, so that the impedance converter 102 can The voltage value of the input signal exceeds the threshold voltage VT of the N-type MOS transistor 104, and the signal missing of the output of the impedance converter 102, that is, the output of the D / A converter according to the second embodiment is prevented. Can do.
[0026]
(Embodiment 3)
Hereinafter, a D / A converter according to Embodiment 3 of the present invention will be described with reference to the drawings. The D / A converter according to the third embodiment has a simpler configuration, and the offset voltage is applied to the analog output of the R-2R ladder resistance type D / A converter as with the D / A converter according to the second embodiment. Is to give.
[0027]
FIG. 7 is a circuit diagram showing a configuration of the D / A converter according to the third embodiment. Note that the configuration and operation of the impedance conversion means 102 are the same as those in the first embodiment, and a description thereof will be omitted.
In FIG. 7, the R-2R ladder resistance type D / A conversion means 210 includes resistors 201, 202a to 202c, 203a to 203d, switches 1011a to 1011d, and a switch control 1010. The reference voltage VREFH is the power supply voltage VDD, and the reference voltage VREFL is the ground voltage. The resistors 202a to 202c are connected in series from the output of the R-2R ladder resistor type D / A converter 210, and the resistor 203a is connected to the connection point of each resistor and both ends of the resistor group including the resistors 202a to 202c. To 203d are connected to each other. Up to this point, it is the same as the normal R-2R ladder resistance type D / A conversion means, but in the R-2R ladder resistance type D / A conversion means 210 according to the third embodiment, one end is a voltage of the reference voltage VREFH. This is different from a normal one in that a resistor 201 is connected to the source (VDD) and the other end is connected to the output of the R-2R ladder resistor type D / A converter 210. By having the resistor 201, an offset is given to the analog signal output S31 of the R-2R ladder resistor type D / A converter 210, and one end of the resistor is connected without preparing the power supply VDD2. An offset can be given by a simple method of changing.
[0028]
In the case of the R-2R ladder resistance type D / A conversion means 210 shown in FIG. 7, when the digital signal is “0000”, the analog signal output S31 becomes the minimum value, but the minimum value is VDD × (43 / 128), which is about 1/3 of VDD. Therefore, the value of VDD may be three times or more the threshold voltage VT of the N-type MOS transistor 104. For example, when 3.3V is used as VDD and the threshold voltage VT is 0.7V, the offset of the analog signal output S31 of the R-2R ladder resistance type D / A converter 210 causes the analog signal output S31 to It can be seen that the minimum value sufficiently exceeds the threshold voltage VT.
[0029]
Thus, according to the D / A converter according to the third embodiment, the R-2R ladder resistor type D / A converter 210 is connected in series from the output of the R-2R ladder resistor type D / A converter 210. A plurality of resistors 202a to 202c, a connection point between the resistors 202a to 202c, and a plurality of resistors 203a to 203d respectively connected to both end points of a resistor group including the plurality of resistors 202a to 202c connected in series; A plurality of switches 1011a to 1011d that are connected to end points of the resistors 203a to 203d that are not on the side of the resistors 202a to 202c and connect the power source voltage (VDD) or the ground voltage (ground) to the resistors 203a to 203d, respectively, A resistor 201 having the other end connected to the output of the R-2R ladder resistor type D / A converter 210 Since the switch control 1010 that controls the plurality of switches 1011a to 1011d is configured based on the input digital signal, the end point of the resistor 201 is connected without preparing the power supply VDD2 as in the second embodiment. With the simple configuration, the offset voltage exceeding the threshold voltage VT of the transistor 104 can be applied to the analog output of the R-2R ladder resistance type D / A conversion unit 210, and the output of the impedance conversion unit 102, ie, D / It is possible to prevent the signal loss of the output of the A converter.
[0030]
(Embodiment 4)
Hereinafter, a D / A converter according to Embodiment 4 of the present invention will be described with reference to the drawings.
FIG. 8 is a block diagram showing the configuration of the D / A converter according to the fourth embodiment. The configuration and operation of the R-2R ladder resistance type D / A conversion means 101 shown in FIG. 8 are the same as those in the first embodiment, and the description thereof is omitted.
[0031]
The impedance conversion unit 300 according to the fourth embodiment includes N-type MOS transistors 104 and 301. The N-type MOS transistor 104 is the same as that of the first embodiment, and the N-type MOS transistor 301 is provided in place of the resistor 103 according to the first embodiment.
[0032]
In the impedance conversion means 102 according to the first embodiment, the N-type MOS transistor 104 and the resistor 103 are used for impedance conversion. However, in this configuration, the resistance component of the transistor 104 and the resistance value of the resistor 103 are manufactured by a semiconductor manufacturing process. Variation in the resistance ratio of the analog output (VOUT) increases, and the final analog output (VOUT) voltage may also vary. However, by using the N-type MOS transistor 301 instead of the resistor 103 as in the impedance conversion unit 300 according to the fourth embodiment shown in FIG. 8, variation in the resistance ratio in the semiconductor manufacturing process can be suppressed. Variations in the output voltage (VOUT) can also be suppressed.
[0033]
(Embodiment 5)
Hereinafter, a D / A converter according to a fifth embodiment of the present invention will be described with reference to the drawings.
FIG. 9 is a block diagram showing the configuration of the D / A converter according to the fifth embodiment. The configuration and operation of the R-2R ladder resistance type D / A conversion unit 101 shown in FIG. 9 are the same as those in the first embodiment, and a description thereof will be omitted.
[0034]
The impedance conversion unit 400 according to the fifth embodiment includes a P-type MOS transistor 401 and a resistor 402. The operation of the impedance conversion unit 400 is substantially the same as that of the impedance conversion unit 102 according to the first embodiment, and a description thereof will be omitted.
[0035]
As described above, even when the impedance conversion means 400 is configured by the P-type MOS transistor 401 as in the D / A converter according to the fifth embodiment, the same effect as that of the D / A converter according to the first embodiment is obtained. Can play.
[0036]
Note that a MOS transistor may be used instead of the resistor 402 as in the case of the fourth embodiment.
[0037]
The transistor constituting the impedance conversion means is not limited to a MOS transistor (MOS-FET), but may be a bipolar transistor or a junction transistor (J-FET). However, when the impedance conversion means is configured by a MOS transistor, the impedance conversion means can be configured on the semiconductor substrate together with the R-2R ladder resistance type D / A conversion means. There is an advantage that can be reduced.
[0038]
【The invention's effect】
As is clear from the above description, according to the D / A converter of the present invention, a digital signal is converted into an analog signal using the first reference voltage and the second reference voltage lower than the first reference voltage. The output impedance of the D / A converter is provided with an R-2R ladder resistance type D / A converter for converting into a signal and an impedance converter for converting the impedance of the analog signal received from the D / A converter. The characteristics can be improved, the output current drive capability can be improved, and the responsiveness to a signal changing at high speed can be improved.
The second reference voltage is a ground voltage, and the R-2R ladder resistance type D / A conversion means has a plurality of first connections connected in series from the output of the R-2R ladder resistance type D / A conversion means. A plurality of second resistors connected to both connection points of the first resistors and both end points of the resistor group consisting of the plurality of first resistors connected in series. A plurality of resistors (each having a resistance value of 2R) and an end point of the second resistor that is not on the first resistor side, and a first reference voltage or a ground voltage is connected to the second resistor. The switch and the second resistor having one end connected to the first reference voltage in common with the plurality of switches and the other end connected to the output of the R-2R ladder resistor type D / A conversion means Based on the different third resistor (resistance value is 2R) and the input digital signal, Switch control means for controlling a plurality of switches, and the impedance conversion means includes an N-type MOS transistor to which an analog signal from the R-2R ladder resistance type D / A conversion means is input to a gate; With a simple configuration having a resistor connected to the N-type MOS transistor, an offset voltage exceeding the threshold voltage VT of the transistor can be applied to the analog output of the R-2R ladder resistance type D / A conversion means. Thus, it is possible to prevent an output signal from the impedance conversion means, that is, an output from the D / A converter, from being lost.
[0039]
According to the D / A converter of the present invention, in the D / A converter, the R-2R ladder resistance type D / A converter provides the offset voltage to the analog signal, so that the impedance conversion is performed. It is possible to obtain an effect of preventing signal loss of the output of the means, that is, the output of the D / A converter.
[0042]
Further, according to the D / A conversion device of the present invention, in the D / A conversion device, the impedance conversion means includes an N-type MOS transistor or a P-type MOS transistor instead of the resistor, so that a semiconductor manufacture is possible. The variation in resistance ratio in the process can be suppressed, and the effect of suppressing the variation in output voltage can be obtained.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a configuration of a D / A converter according to a first embodiment of the present invention.
FIG. 2 is a waveform diagram for explaining the function and effect of the D / A converter according to Embodiment 1 of the present invention.
FIG. 3 is a circuit diagram showing a configuration of a D / A converter according to Embodiment 1 of the present invention.
FIG. 4 is a waveform diagram showing an input waveform and an output waveform to impedance conversion means in the D / A converter according to Embodiment 1 of the present invention.
FIG. 5 is a circuit diagram showing a configuration of a D / A converter according to a second embodiment of the present invention.
FIG. 6 is a waveform diagram showing an input waveform and an output waveform to impedance conversion means in the D / A converter according to Embodiment 2 of the present invention.
FIG. 7 is a circuit diagram showing a configuration of a D / A converter according to a third embodiment of the present invention.
FIG. 8 is a circuit diagram showing a configuration of a D / A converter according to a fourth embodiment of the present invention.
FIG. 9 is a circuit diagram showing a configuration of a D / A converter according to a fifth embodiment of the present invention.
FIG. 10 is a circuit diagram showing a configuration of a conventional D / A converter.
FIG. 11 is a waveform diagram for explaining the function and effect of a conventional D / A converter.
[Explanation of symbols]
100 D / A conversion device 101, 210 R-2R ladder resistance type D / A conversion means 102, 300, 400 Impedance conversion means 103, 201, 202a to 202c, 203a to 203d, 402 Resistance 104, 301 N-type MOS transistor 105 Low-pass filter 401 P-type MOS transistor 1010 Switch control 1011a, 1011b, 1011c, 1011d switch

Claims (4)

R-2R ladder resistance type D / A conversion means for converting a digital signal into an analog signal using a first reference voltage and a second reference voltage lower than the first reference voltage;
Impedance conversion means for converting the impedance of the analog signal received from the D / A conversion means;
With
The second reference voltage is a ground voltage,
The R-2R ladder resistance type D / A conversion means is:
A plurality of first resistors (each resistance value is R) connected in series from the output of the R-2R ladder resistance type D / A conversion means;
A plurality of second resistors (each resistance value is 2R) respectively connected to each connection point of the first resistors and both end points of a resistor group composed of the plurality of first resistors connected in series;
A plurality of switches that are respectively connected to end points that are not on the first resistance side of the second resistor, and that connect a first reference voltage or a ground voltage to the second resistor;
A third resistor different from the second resistor, having one end connected to the first reference voltage in common with the plurality of switches and the other end connected to the output of the R-2R ladder resistor type D / A converter. Resistance (resistance value is 2R),
Based on the input digital signal, it has a, a switch control means for controlling said plurality of switches,
The impedance converting means includes
An N-type MOS transistor to which an analog signal from the R-2R ladder resistance type D / A converter is input to the gate;
A resistor connected to the N-type MOS transistor;
A D / A converter characterized by the above. The D / A converter according to claim 1,
The R-2R ladder resistance type D / A conversion means applies an offset voltage to the analog signal. The D / A converter according to claim 1 ,
The D / A converter according to claim 1, wherein the impedance conversion means includes a P-type MOS transistor instead of the N-type MOS transistor. The D / A converter according to claim 1 or 3 ,
The D / A conversion device characterized in that the impedance conversion means includes an N-type MOS transistor or a P-type MOS transistor instead of the resistor.

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