JPH0464210B2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field of the Invention) The present invention relates to a successive approximation type analog-to-digital converter using a capacitance ladder, and particularly to a feed back switch used in the analog-to-digital converter. be.

(Technical Background) Figure 1 shows a conventional capacitive ladder successive approximation analog-to-digital (hereinafter abbreviated as AD) conversion circuit.

A successive approximation type AD conversion circuit using capacitance is 2 ⁿ
The capacity weighted to (0 or a positive integer) is
By using the fact that charging is performed (sample-and-hold) based on the potential difference between a certain reference voltage and an analog potential, and the amount of accumulated charge at that time does not change,
This is a method of successive conversion.

At this time, the offset voltage of the comparator that successively compares with a certain reference potential affects the conversion accuracy.
In order to cancel the effect of this offset voltage, 1
First, it uses two operational amplifiers (hereinafter referred to as operational amplifiers) as a voltage follower to charge and discharge the capacitance. Next, the feed back loop is cut off using an analog switch (hereinafter referred to as the feed back switch).
The above operational amplifier is used as a comparator to compare potentials. Therefore, even if an offset voltage occurs, since the same operational amplifier is used, it will be canceled out, making it possible to perform AD conversion with high accuracy.

However, in the above system, leakage current in the feedback switch for interrupting the feedback loop in the operational amplifier poses a problem.

In the case of AD converters that use capacitors, maintaining the amount of accumulated charge during charging and discharging (sample-and-hold) until the end of conversion is the most important point for improving accuracy. This greatly affects conversion accuracy.

When the feedback switch is turned off, the leakage current increases as the potential difference across the feedback switch increases.

When an operational amplifier is used as a comparator, the output level of the operational amplifier changes between the power supply voltage and ground. On the other hand, when an operational amplifier is used as a comparator, the input level on the capacitor side of the operational amplifier is determined by the analog potential level during charging and discharging (sample and hold), so the input level is the maximum and minimum, respectively. In the case of , the potential difference across the feedback switch is maximum. Therefore, in conventional AD conversion circuits, a leakage current corresponding to the analog input level of the AD conversion circuit begins to flow, changing the amount of stored charge that is charged and discharged, and deteriorating conversion accuracy.

(Objective of the Invention) The object of the present invention was to eliminate the above-mentioned drawbacks observed in conventional AD conversion circuits, and to prevent changes in the amount of accumulated charge, that is, current leakage, and to improve conversion accuracy. The object of the present invention is to provide an AD conversion device with good performance.

(Summary of the Invention) This invention uses a plurality of feed back switches in an operational amplifier, and the first feed back switch SW6 closest to the end of the capacitance ladder (first capacitance ladder circuit) When in the cut-off state, this first feed back
A feature of this switch is that it prevents leakage of accumulated charges by eliminating the potential difference between both ends of the switch.

(Function of the Invention) The present invention is configured to prevent the charges accumulated in the capacitive ladder group from leaking to the outside when comparing the charges accumulated in the capacitive ladder group with a reference voltage.

A successive approximation type AD conversion circuit using an operational amplifier uses the operational amplifier as both a portage follower and a comparator in order to cancel out the influence of the operational amplifier's offset voltage. The problem here is when an operational amplifier is used as a comparator. In other words, at the output terminal of the operational amplifier,
While the power supply voltage or the ground voltage appears alternately, a voltage corresponding to an analog signal voltage defined within the range of the power supply voltage and the ground voltage appears at the inverting input terminal. Therefore, a potential difference always occurs between the output terminal and the inverting input terminal of the operational amplifier (this is particularly noticeable at the beginning of the successive conversion operation), and the potential difference is particularly maximum when the analog signal voltage is at its maximum and minimum. Therefore, switch SW6 in Figure 1
A large potential difference occurs between both ends of the capacitor, and the charge in the capacitor ladder leaks to the outside via the switch SW6.

Therefore, the present invention is constructed based on the idea that this leakage will not occur if the voltages appearing across the switch SW6 are made equal when the operational amplifier is used as a comparator. That is, the first feed back switch
The other end of SW6 (output terminal side of the operational amplifier) is electrically insulated by a second field back switch SW7 so as not to be affected by large potential changes occurring at the output terminal of the operational amplifier, and further
At the other end of SW6, a second capacitance ladder weighted with the same capacitance ratio as the capacitance ladder group (first capacitance ladder group) that supplies a voltage equal to the voltage appearing at one end of SW6 (the inverting input terminal of the operational amplifier). It is a connection of groups. With this configuration, even if the output terminal of the operational amplifier makes a full swing between the power supply voltage and the ground voltage, no leakage occurs in SW6, so the charge in the capacitive ladder circuit is maintained without leaking to the outside. Ru.

Here, due to the influence of the large potential change that occurs at the output terminal of the operational amplifier that is in full swing,
A leak occurs in SW7 and the second
Some charge from the capacitance ladder group escapes, and SW
The voltage at the other end of 6 fluctuates somewhat. but,
The voltage at the other end of SW6 does not fluctuate greatly like the output terminal of the operational amplifier, and only the voltage supplied from one end of the second capacitance ladder group (the other end of SW6) fluctuates slightly. Therefore, SW
Since a potential difference large enough to cause leakage to SW 6 does not occur between one end and the other end of SW 6, the charge in the first capacitor ladder group is held. In addition, in this series of operations, as mentioned above, due to leakage occurring in SW7, some of the charge in the second capacitance ladder group escapes, but the role of this second capacitance ladder group is When the first capacitor ladder group performs successive conversion operation, a voltage that does not cause leakage to SW6 is supplied to the other end of SW6. There is no problem because the charge does not escape.

(Embodiments of the invention) Examples of the invention will be described below with reference to the accompanying drawings. FIG. 2 is a circuit diagram showing a first embodiment of the present invention. In the figure, the inverting input terminal of the operational amplifier has 2 ³ C ₀ , 2 ² C ₀ ,
2 ¹ C ₀ , 2 ⁰ C ₀ , 2 ⁰ C ₀ , and the first
The other end of the first capacitor group is connected to the switches SW1 to SW4 that switch between the A terminal side of the switch SW5 and the reference voltage _VR side. However, the least significant bit, 2 ⁰ C ₀
Any one of them is directly connected to the A terminal of the switch SW5 without going through any switch. Further, a potential corresponding to 1/2 of the reference voltage V _R is applied to the non-inverting input terminal of the operational amplifier. The output terminals of the operational amplifier are two switches SW6, which are field effect transistors.
The two switches SW6 and SW7 are connected to the inverting input terminal of the operational amplifier via SW7.
Simultaneously turns on and off depending on the voltage applied to terminal 3
control.

Between switch SW6 and switch SW7,
A second capacitance group weighted by the same capacitance ratio as the first capacitance group 2 ³ C ₁ , 2 ² C ₁ , 2 ¹ C ₁ , 2 ⁰ C ₁ , 2 ⁰ C ₁
is connected to the other end of the second capacitor group, and similarly to the first capacitor group, switches SW8 to SW5 are connected to switch SW5 to switch between the A terminal side and the reference voltage V _R side.
SW11 is connected, _and one of the least significant bits, ^20C1 , is connected to the A terminal of switch SW5. Note that the switch SW5 is a switch for switching between the analog input terminal V _IN and the ground. Further, the output terminal of the operational amplifier is connected to one end of the switch SW7, and at the same time, via the terminal 2, to a comparison result latch and a control circuit of each switch group.

Next, the operation of the AD conversion circuit will be explained.

First, connect switch SW5 to analog input terminal V _IN
SW1 to SW4 and SW8 to SW11 connected to each capacitor group are connected to the analog input terminal V _IN side in order to apply an analog input level to each of the first and second capacitor groups. At that time, the switches SW6 and SW7 are rendered conductive, the operational amplifier functions as a voltage follower, and each capacitor group is charged and discharged by the potential difference between 1/2V _R [V] and the analog input voltage V _IN [V].

After charging and discharging each capacitor group, switch SW
6. Cut off switch SW7, connect switch SW5 to the ground side, and the circuit enters the hold state.

Then, to perform a most significant bit comparison,
Switch SW1 and connect the switch side terminal at 2 ³ C ₀ to the reference voltage _VR . At this time, the first
Charge moves within the capacitance group, and as a result, the potential of the inverting input terminal of the operational amplifier changes, and that potential and the potential of 1/2 of the reference voltage V _R [1/2
V _R ], the most significant bit (MSB) is determined, and if the signal output from the output terminal of the operational amplifier used as a comparator is at “HIGH” level, switch SW1 is connected to the reference voltage V _R terminal. side, and if it is “LOW” level, switch SW
Connect 1 to the ground side. Thereafter, similarly, the second and third bits are successively compared to determine the switch states of switches SW1 to SW4. This state becomes the digital conversion output and AD
A series of circuit operations for conversion is completed.

To perform the above conversion operation, input the output result from the operational amplifier that functions as a comparator from terminal 2, and switch SW1 connected to the first capacitor group.
~Control signal 1B that turns switch SW4 ON and OFF
~4B is generated, but this control signal is
is similarly input to the capacitance group, and the potential between switch SW6 and switch SW7 is brought close to the potential at the inverting input terminal of the operational amplifier functioning as a comparator. In other words, the difference between both ends of switch SW6 is reduced. The control signals 1B to 4B of the switches SW8 to SW11 in the second capacitance group are the same as the signals input to the switches SW1 to SW4 in each of the first capacitance groups, and are given as weights 2 ³ ,
The capacitors corresponding to 2 ² , 2 ¹ , and 2 ⁰ are connected and switched between the reference voltage V _R and the ground in the same way as in the first and second capacitor groups.

For the above-mentioned reason, when the first switch SW6 closest to the end of the capacitance ladder of C ₀ is in the cut-off state, the potential difference between both ends of the first switch SW6 can be made almost zero.

Furthermore, since it is sufficient that the weights are given to each capacity group at the same ratio, it is not necessary to make the absolute values of each capacity group equal, and the unit capacity in Fig. 2 is C ₁
It is possible to design the second capacitor group with a smaller pattern area than the first capacitor group with a unit capacitance of C ₀ .

There is a limit to the area of unit capacitance that can be reduced or miniaturized due to constraints such as noise margin, precision, etc., so sufficient consideration must be given when setting the capacitance value in the first capacitance group. . However, in setting the capacitance value in the second capacitance group according to the present invention, compared to setting the capacitance value in the first capacitance group, there is less influence on accuracy.
The capacitance value can be set with low precision. This is because the second capacitor group is not used for the purpose of AD conversion using the accumulated charge amount, but is used to reduce the unit difference between both ends of the feedback switch in the operational amplifier. , the unit capacitance value in the second capacitance group does not need to be the same as the unit capacitance value in the first capacitance group, and if the ratio of the weighted capacitances is equal, the unit capacitance value in the first capacitance group By setting the capacitance value smaller than , the pattern area can be reduced.

FIG. 3 is a circuit diagram showing a second embodiment of the present invention. In this figure 3, the switch
SW4 and SW11 are switches for switching between the common example of switch SW5, which switches between the analog input terminal V _IN and ground (ground), and a potential [1/2V _R ] that is 1/2 of the reference voltage _VR . The connection reduces the ladder capacity by half in this second embodiment. The rest is completely the same as the first embodiment shown in FIG.

(Effects of the Invention) As explained above, the present invention provides that when the first switch closest to the capacitance ladder end of the feedback switch connected to the operational amplifier is cut off, the first switch By applying the voltage to the terminal opposite to the terminal connected to the end of the capacitor ladder from another voltage source (consisting of a group of capacitors with similar weights), the potential difference across the first switch is increased. This has the excellent advantage that it is possible to minimize the leakage current when the first switch is cut off, and it is possible to obtain an AD conversion circuit with high accuracy.

In the case of a semiconductor integrated circuit, in order to reduce the leakage current of this first switch, the pattern area of the switch is reduced. Increase the switch threshold voltage. Therefore, for example, the impurity concentration under the gate oxide film is reduced. Methods such as the following are possible, but since the resistance component of the switch becomes large in both methods, a long time is required to charge and discharge the capacitor group, and high-speed AD conversion cannot be performed. This is not a good idea because it has drawbacks such as a large offset voltage in the operational amplifier. However, in this invention, the leakage current in the first switch can be minimized without modifying the feedback switch itself or taking process measures to control the impurity concentration. It has the advantage of being suppressable.

As the resolution in AD conversion increases, the conditions for increasing the conversion accuracy become stricter.
The influence of the amount of leakage of the stored charge amount on the conversion accuracy increases. In other words, the conversion accuracy is determined by this leakage amount. using a capacity ladder
In the case of an AD conversion circuit, the purpose is to cut offset voltage using the configuration shown in Figure 1, so the leakage path for the amount of charge stored in the capacitor is the feed back switch portion and the substrate. Most of them are. This invention provides an excellent effect in that the leakage current of the feedback switch can be suppressed to a minimum.

[Brief explanation of the drawing]

Figure 1 shows the conventional capacitance ladder method successive approximation type
FIGS. 2 and 3 are circuit diagrams showing a circuit of an AD converter. FIGS. 2 and 3 are circuit diagrams showing a circuit of a capacitive ladder type successive approximation AD converter according to an embodiment of the present invention. 1, 2, 3...terminal, R...resistance, SW1~
SW11...Switch.

Claims (1)

[Claims] 1 Consists of a unit capacitance and a plurality of capacitors weighted by a capacitance ratio of 2 ^n-1 times (where n is a natural number) the unit capacitance, one end of each capacitor being connected in common. a first switch that selectively outputs an analog signal voltage level and a ground voltage level; and a first switch connected to each other end of the capacitor except for the unit capacitance, and a first switch that selectively outputs an analog signal voltage level and a ground voltage level; a second switch group consisting of a plurality of switches that selectively transmit the voltage level output from the first switch or the first reference voltage level; and an inverting input terminal, a non-inverting input terminal, and an output terminal. an operational amplifier having the inverting input terminal connected to one end of the capacitor group and the non-inverting input terminal connected to a second reference voltage source; and in response to the output of the operational amplifier, the second switch a switch switching circuit that outputs a switching signal for switching each switch in the group; and a switch switching circuit having one end and the other end, the one end being connected to one end of the capacitor group, and the other end being connected to one end of a fourth switch. a fourth switch having one end and the other end, the one end being connected to the other end of the third switch, and the other end being connected to the output terminal of the operational amplifier; voltage supply means connected to the other end of the third switch and supplying the other end of the third switch with a voltage level equal to the voltage level appearing at one end of the capacitor group; turning on the third and fourth switches during a first period in which the switch is outputting the analog signal voltage level;
During a second period in which the first switch is outputting the ground voltage level, turn off the third and fourth switches and detect the state of the second switch that has changed in response to the switching signal. A capacitive ladder type analog-to-digital converter, characterized in that a digital conversion output of the analog signal voltage level is obtained by performing the following steps.