JP2878606B2 - D / A conversion circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a D / A conversion circuit using a weighted impedance.

[0002]

2. Description of the Related Art FIG. 8 shows a conventional D / A conversion circuit using a weighted impedance disclosed in Japanese Patent Application Laid-Open No. 4-150229. In the figure, 1 and 2 are input terminals, 3 and 4 are output terminals, 5 to 7 and 8 to 10 are resistors,
Switches 11 to 13 each have a b-contact (normally closed contact) for controlling whether or not both ends of the resistors 5 to 7 are short-circuited.
(B contact switch) 14 to 16 are a contacts (normally open) for controlling whether or not both ends of the resistors 8 to 10 are short-circuited.
A switch (a contact switch) having a contact (a contact) , a circuit # 1 is configured by the resistors 5 to 7 and the switches 11 to 13, and a circuit # 2 is configured by the resistors 8 to 10 and the switches 14 to 16 . Reference numerals 32, 19, and 20 denote switch drivers that control on / off of the switches 11 to 16, and reference numeral 21 denotes a control circuit that outputs control signals 29 to 31 to the switch drivers 32, 19, and 20.

Next, the operation will be described. First, control signals 29 to 31 are output from the control circuit 21. When the signal is input to the switch drivers 32, 19, and 20,
The switch driver 32 turns on and off the switches 11 and 14, the switch driver 19 turns on and off the switches 12 and 15, and the switch driver 20 turns on and off the switch 1.
On and off of 3, 16 are controlled. Here, switch 1
1 to 13 are b-contact switches, and switches 14 to 16
Is an a contact switch, so that the resistors 5 and 8 and the resistor 6
And both ends of the resistor 9 and only one of the resistors 7 and 10 are short-circuited. Thus, the circuit # 1 and the circuit # 2
, A voltage obtained by dividing the voltage between the input terminals 1 and 2 into eight values is obtained between the output terminals 3 and 4.

[0004]

In the conventional circuit as described above, all the switches are momentarily turned on due to the variation of the switches, so that the input terminals 1 and 2 are short-circuited and an overcurrent flows. As a result, the resistor and the switch may be damaged. For example, the control signal 29
To 31 change from "L, L, L" to "H, H, H", the return time of all switches of the circuit # 1 is changed to the circuit # 2.
If it is faster than the reset time of all switches, input terminal 1
And 2 are short-circuited. Also, control signals 29 to 31
Changes from “H, H, H” to “L, L, L”,
If the operation time of all the switches of the circuit # 2 is faster than the operation time of all the switches of the circuit # 1, the input terminals 1 and 2
There is a problem that short circuit occurs between the two.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and by providing a resistor between the input terminals 1 and 2 that is not short-circuited at both ends, the input terminal An object of the present invention is to prevent a switch or a resistor from being damaged due to a short circuit between 1 and 2, and to improve the reliability of a D / A conversion circuit using a weighted impedance.

[0006]

A D / A conversion circuit according to the present invention has a connection between a first input terminal and a first output terminal.
Connected by connecting n weighted resistors in series.
1 resistor group, the first output terminal and the second input terminal.
Between the first resistor group and the second resistor group.
2 and a resistor group connected in parallel to each resistor of the first resistor group.
A first switch having a continuous b-contact,
A having an a-contact connected in parallel with each resistor of the resistor group
2 switch, any of the above first or second switch
By closing one and opening the other
Switched to control to short both ends of the resistor
Driver, both ends of the above resistors are all short-circuited at the same time
Protection means for preventing the first and second inputs
The potential obtained by dividing the voltage between terminals by the resistor is
1 output terminal and the same potential as the second input terminal.
It is configured to output between the second output terminal .

Further, the protection means is connected to a first switch connected to one resistor of the first resistor group and one resistor of a second resistor group corresponding to the resistor. and a second switch, in which have configured <br/> in switch having contacts of two pairs.

The protection means may be configured such that when the signal for controlling the switch changes, the b-contact switch connected to one of the first resistor group is turned off within the off-time, and The delay circuit includes an ON time of an a-contact switch connected to one of the second resistor group corresponding to the resistor.

Further, when the signal for controlling the switch is changed, the protection means is provided within the time from the start of turning off the b-contact switch connected to one of the first resistors to the end of turning off. A microprocessor unit (MPU) for controlling the ON time of the a-contact switch connected to one of the second group of resistors corresponding to the resistors.

Further, as a protection means in the event of a power failure in the D / A conversion circuit, the time required for the b-contact switch connected to one of the first resistors to return to the on-state is determined by the return time. A voltage drop detection circuit is provided for shortening the time required for the a-contact switch connected to one of the two resistors to return to the off state.

[0011]

The D / A conversion circuit according to the present invention controls the operation time and the recovery time of the switch, so that both ends are not necessarily short-circuited in the resistor group even when the control signal of the switch changes or the power is cut off. The presence of the resistor prevents the input terminals from being short-circuited and the switch and the resistor from being damaged by a large current.

[0012]

【Example】

Embodiment 1 FIG. FIG. 1 shows an embodiment of the present invention.
Reference numeral 3 denotes a so-called 2c contact switch having two pairs of contacts and one pair serving as the other back contact. One of the c contacts is connected in parallel with the resistor 5 and the other one is connected. The c contact is connected in parallel with the resistor 8. FIG. 2 shows the connection between the resistors 5 and 8 and the 2c contact switch 33. Other configurations are the same as those of the conventional device shown in FIG.
Since the 2c contact switch 33 controls two c contacts by one coil, the two c contacts are turned on and off at the same time. Therefore, as shown in FIG. 2, when the resistors 5 and 8 are connected to the 2c contact switch 33 such that when both ends of one resistor are short-circuited, both ends of the other resistor are not short-circuited. Are not simultaneously short-circuited, a short circuit between the input terminals 1 and 2 can be prevented, and a highly reliable D / A conversion circuit can be obtained.

Embodiment 2 FIG. 3 uses a weighted impedance provided with a protection circuit for turning on one of the switches connected to the resistors 5 and 8 completely and then turning on the other switch to further increase the reliability. This is an example of a 3-bit D / A conversion circuit. In the figure, a protection circuit 22 is a delay circuit 23 for delaying a control signal 29, an inverter 24 with a Schmitt for correcting a dull output from the delay circuit 23, and two inverters 25 and 26;
The circuit includes an AND circuit 27 that generates an input signal to the switch driver 18 and a NAND circuit 28 that generates an input signal to the switch driver 17. Other configurations are the same as those in FIG.

As shown in FIG. 4, first, a control signal 29 is output. When the control signal 29 changes from "L" to "H", at least one input of the NAND circuit 28 becomes "L" via the inverter 26, "H" is input to the switch driver 17, and the b-contact switch 11 turns off. The control signal 29 is delayed by the delay circuit 23 and shaped by the Schmitt-equipped inverter 24.
The signal is input to the AND circuit 27 via the inverter 25.
Since the other input of the AND circuit 27 is already "H", the AND circuit 27 outputs "H", "H" is input to the switch driver 18, and the a-contact switch 14
Turns on (later by time 34). In this way,
The operation time of the a-contact switch 14 is later than the operation time of the b-contact switch 11.

Next, when the control signal 29 changes from "H" to "L", one input of the AND circuit 27 becomes "L", "L" is input to the switch driver 18, and the switch 14 is turned off. I do. Further, the control signal 29 is the delay circuit 2
3, the signal is shaped and inverted by the Schmitt-equipped inverter 24, and “H” is input to the NAND circuit 28. Since the other input of the NAND circuit 28 is already at “H” via the inverter 26,
The D circuit 28 outputs "L", "L" is input to the switch driver 17, and the switch 11 is turned on (with a delay of time 35). Thus, the return time of the contact b is a
It is slower than the contact recovery time. From the above, the control signal 29
The timing of the operation of the contact switch 14 and the operation of the contact switch 11 are as shown in FIG. Therefore, the resistor 5 and the resistor 8 are not short-circuited at the same time, and the input terminal 1
2 can be prevented.

Embodiment 3 FIG. 5 shows a weighted impedance 3-bit D / A conversion circuit provided with a protection circuit using a voltage drop detection circuit. FIG.
In the conventional D / A conversion circuit using the weighted impedance shown in FIG. 1, when power is cut off when the b contact switches 11 to 13 are off and the a contact switches 14 to 16 are on, If the return time of all the b-contact switches is faster than the return time of all the a-contact switches, the input terminals 1 and 2 are short-circuited. In the third embodiment, a protection circuit 36 including a voltage drop detection circuit 37 and an AND circuit 38 is provided in order to eliminate the above-mentioned inconvenience. A voltage drop is detected before the switch returns due to the drop, and "L" is output. "L" is input to the AND circuit 38, "L" is input to the switch driver 18, and the a-contact switch 14 is turned off. I do. in this way,
By providing the protection circuit 36, the a-contact switch 14 returns before the b-contact switches 11 to 13 return when the power is turned off, so that a short circuit between the input terminals 1 and 2 can be prevented.

Embodiment 4 FIG. FIG. 6 shows a 3-bit D / A using weighted impedance protected by software.
FIG. 7 is a flowchart of software processing performed by the MPU 39, which is an example of a conversion circuit. In FIG. 6, when the control signal 29 changes from “L” to “H”, one input of the OR circuit 40 becomes “H” and the switch driver 1
Since “H” is input to 7, the b-contact switch 11 is turned off. On the other hand, the other input signal of the AND circuit 41 is delayed according to the flowchart of FIG. 7, and “H” is input after n seconds. Since the other input of the AND circuit 41 is already at “H”, the AND circuit 41 outputs “H”, and “H” is input to the switch driver 18, so that the a-contact switch 14 is turned on. . here,
If the operating time of the b-contact switch 11 is set to be shorter than n seconds, the b-contact switch 11 is always turned off and then the a-contact switch 14 is turned on.

When the control signal 29 changes from "H" to "L", one input of the AND circuit 41 becomes "L" and "L" is inputted to the switch driver 18, so that the a contact switch 14 turns off. On the other hand, OR circuit 4
0 is input according to the flowchart of FIG.
Seconds later, since “L” is input and “L” is already input to the other input, the OR circuit 40 outputs “L”,
“L” is input to the switch driver 17, and the b-contact switch 11 is turned on. If the return time of the a-contact switch 14 is shorter than m seconds, the a-contact switch 14 is always turned off and then the b-contact switch 11 is turned on. As described above, the resistors 5 and 8 are not short-circuited at the same time, and a short circuit between the input terminals 1 and 2 can be prevented.

The above operation will be described with reference to the flowchart of FIG. First, the control signal 2 output from the control circuit 21
9 is read (S1). The control signal ("H" or "L") read in S1 is stored in A (S2). Next, the control signal 29 currently output by the control circuit 21 is read (S3). The control signal (“H”) read in S3
Alternatively, “L”) is stored in B (S4). The above A and B are compared (S5). If A = B, the control signal 29 has not changed, and the MPU 39 returns to S3 without performing any control. If A <B, the control signal has changed from “L” to “H”, so the MPU 39 starts n seconds (S6). If A> B, the control signal 29 has changed from “H” to “L”, so the MPU 39 starts a m-second start (S9).

When the control signal 29 changes from "L" to "H", in order to prevent a short circuit between the input terminals 1 and 2 in FIG. Just turn on. Therefore, start n seconds S
In step 6, n seconds are counted (where n> b is the operating time of the contact switch). When the n-second count ends (S
7) The process proceeds to S8, and “H” is output from the MPU 39.

When the control signal 29 changes from "H" to "L", in order to prevent a short circuit between the input terminals 1 and 2 in FIG. Just turn on. Therefore m seconds start S
In step 9, m seconds are counted (where m> a is the return time of the contact switch). When the m-second count ends (S
10) The process proceeds to S11, and “L” is output from the MPU 39.

Further, the current state of the control signal 29 stored in B is written in A (S12), and thereafter, the process returns to S3, the next state is stored in B, and A and B are compared again. The above operation can be realized by performing the above control by the MPU 39.

In each of the above embodiments, the switch is described as a switch having a mechanical configuration. However, the same effect can be obtained by using a switch having an electrical configuration that performs the same operation. Needless to say, the present invention includes a device using a switch having such an electrical configuration.

[0024]

As described above, according to the present invention, in the D / A conversion circuit using the weighted impedance, the means for preventing the short circuit between the input terminals is provided, so that the reliability can be improved.

Further, by using a 2c contact switch as a means for preventing a short circuit between input terminals, a reliable operation can be guaranteed with a simple configuration.

Also, as a means for preventing a short circuit between the input terminals, a delay circuit or an MPU is used to turn on the b-contact switch after the a-contact switch is turned off and to turn on the a-contact switch after the b-contact switch is turned off. By turning on the contact switch, a short circuit between the input terminals can be prevented when the switch control signal changes, and the reliability can be improved.

Further, by providing a circuit for detecting a voltage drop when the power is turned off, a short circuit between input terminals can be suppressed even in the event of an accident, and damage to switches and resistors can be prevented.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a D / A conversion circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a part of FIG. 1 in detail.

FIG. 3 is a circuit diagram illustrating a D / A conversion circuit according to a second embodiment of the present invention.

FIG. 4 is a waveform diagram illustrating the operation of FIG.

FIG. 5 is a circuit diagram showing a D / A conversion circuit according to Embodiment 3 of the present invention.

FIG. 6 is a circuit diagram showing a D / A conversion circuit according to Embodiment 4 of the present invention.

FIG. 7 is a flowchart illustrating the operation of FIG. 6;

FIG. 8 is a circuit diagram showing a conventional D / A conversion circuit.

[Explanation of symbols]

1, 2, input terminal, 3, 4 output terminal, 5, 6, 7,
8, 9, 10 resistance, 11, 12, 13 b contact switch, 14, 15, 16 a contact switch, 17, 18,
19, 20 switch driver, 21 control circuit, 22
Protection circuit, 23 delay circuit, 24, 25, 26 inverter, 27 AND circuit, 28 NAND circuit, 29,
30, 31 control signal, 32 switch driver, 33
2c contact switch, 36 protection circuit, 37 voltage drop detection circuit, 38 AND circuit, 39 MPU, 40 O
R circuit, 41 AND circuit.

Claims (5)

(57) [Claims] A first resistor group connected between a first input terminal and a first output terminal, the first resistor group being formed by connecting n weighted resistors in series; A second resistor group connected between the second resistor and the second resistor, and configured in the same manner as the first resistor group, in parallel with the respective resistors of the first resistor group;
A first switch having a b-contact connected to
A contact connected in parallel with each resistor of
A second switch, a switch driver that closes one of the first and second switches and opens the other to short-circuit both ends of one of the resistors, and both ends of each of the resistors. And a protection means for preventing all of them from being short-circuited at the same time. A potential obtained by dividing the voltage between the first and second input terminals by the resistor is supplied to the first output terminal and the second input terminal. A D / A converter circuit configured to output between a terminal and a second output terminal having the same potential. 2. The protection means is connected to a first switch connected to one of the first group of resistors and to one resistor of a second group of resistors corresponding to the first switch. a second switch, according to claim 1 Symbol placement of the D / a converter circuit, characterized in that is constituted by a switch having contacts of two pairs. 3. The protection means, when a signal for controlling the switch changes, within a time from the start of turning off the first switch connected to one of the first resistors to the end time of turning off the first switch. 2. The D / A conversion circuit according to claim 1 , wherein the D / A conversion circuit is constituted by a delay circuit for storing an ON time of a second switch connected to one of the second resistor group corresponding to the resistor. 4. The protection means, when a signal for controlling the switch changes, within a time from the start of turning off the first switch connected to one of the first resistors to the end time of turning off the first switch. according to claim 1, characterized in that it has a microprocessor unit (MPU) that controls so as to keep the second switch on-time, which is connected to one resistor of the second resistor group corresponding to the resistance D / A conversion circuit. 5. The protection means according to claim 1, wherein a voltage drop detection circuit detects a voltage drop when a power failure occurs in the D / A conversion circuit, and the protection means is connected to one of the first resistance groups. claim 1 than the recovery time of the on-the first switch, characterized by being configured so as to speed up the return time to the second switch-off which is connected to one resistor of the second resistor group The D / A conversion circuit according to the above.