JPH02159826A - A/d converter - Google Patents

Abstract

PURPOSE:To reduce the conversion error by providing plural switches and using the same reference voltage through the control of a control circuit for integration and discharge and using the analog input signal as an input signal of a comparator directly. CONSTITUTION:Let the period of the clock be T and an output of an A/D converter be k-bit(k is an integral number), then switches 7, 8 are at first turned off and switches 9, 10 are turned on to integrate a signal in response to the reference voltage VREF, till a counter 6 counts signals by N(=2<k>), where NT is an integration period. Then the switches 7, 8 are turned on and the switches 9, 10 are turned off to apply discharge (nT is a discharge period) by the voltage VREF, and a crossing time of the output of an amplifier 1 with the input level is discriminated by a comparator 2 and the control circuit 5 stops the counter 6. Then the content of the counter 6 counting down from the start of discharge till the stop of the counter 6 is an output of A/D converter. Thus, the switches 7-10 are used in this way to control them from the circuit 5 thereby reducing the conversion error.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an A/D conversion circuit, and particularly to an integral type A/D conversion circuit in an LSI.

[Conventional technology]

In conventional LSI circuits, A/D conversion circuits that convert analog signals to digital signals, especially integral type A/D conversion circuits, are A/D conversion circuits that use amplifiers that amplify and integrate reference voltages, resistors, and RC circuits of capacitors. /D conversion circuits are known. FIG. 4 is a circuit diagram of a precision A/D conversion circuit showing an example of such a conventional method.

As shown in FIG. 4, the integral type A/D conversion circuit is driven by an amplifier 1 and a comparator 2 that amplify the reference voltage VREF, a capacitor 3 and a resistor 4 that integrate the reference voltage, the comparator 2 and a clock.
It has a control circuit 5 that controls the opening and closing of switches 7.8 connected to the R, terminal and analog input terminal, respectively, and a counter 6 that counts and outputs digital signals from the control circuit 5. Next, a conversion operation will be described when the clock cycle of the A/D conversion circuit is T and the digital output is set to bits (k: integer).

FIG. 5 is an output voltage characteristic diagram of the amplifier in FIG. 4.

As shown in FIG. 5, first turn off the switch 7 and turn on the switch 8, and integrate according to the input until the counter 6 counts N (=23) (nT is the integration period)
. Next, switch 7 is turned on and switch 8 is turned off to discharge the charge accumulated in the capacitor 3 using the reference voltage VREF (nT is the discharge period), and when the output of the amplifier 1 crosses the ground potential, the comparator 2, and the counting operation of the counter 6 is stopped. The n number counted from the start of discharge until the counter 6 stops becomes the output of A/D conversion.

[Problem to be solved by the invention]

The above-mentioned conventional integral type A/D conversion circuit has a problem in that the input impedance cannot be made high in LSIs where it is difficult to obtain high resistance. That is, if the input impedance is not high, the output impedance of the previous stage relative to the analog input will affect the conversion error, and the difference between the output impedance on the reference voltage side and the output impedance on the analog input signal side will also cause a conversion error.

An object of the present invention is to provide an A/D conversion circuit that reduces such conversion errors.

[Means to solve the problem]

The A/D conversion circuit of the present invention includes a resistor whose one end is connected to a reference voltage terminal, and an amplifier whose (-) side input terminal is connected to the other end of the resistor and whose (+) side input terminal is grounded. , a first switch having one end connected to the (-) side input terminal of the amplifier, a capacitor having one end connected to the other end of the first switch, and one end connected to the other end of the capacitor; a second switch whose other end is connected to the output terminal of the amplifier; and a third switch whose one end is connected to the connection point between the first switch and the capacitor and whose other end is connected to the output terminal of the amplifier. , one end is connected to the connection point of the second switch and the capacitor, and the other end is connected to the (-) of the amplifier.
) @ a fourth switch connected to the input terminal, the (-) side input terminal being connected to the output terminal of the amplifier, and (+)
a comparator whose side input terminal is supplied with an analog voltage from an analog input terminal; a control circuit which controls the first to fourth switches based on a clock and a signal from the comparator; and a control circuit which is connected to a digital output terminal and which controls the first to fourth switches. The circuit includes a counter for counting and outputting digital signals from the control circuit.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is an A/D conversion circuit diagram showing a first embodiment of the present invention.

As shown in FIG. 1, the integral type A/D conversion circuit of this embodiment includes a resistor 4 whose one end is connected to a reference voltage terminal,
The (-) side input terminal is connected to the other end of the resistor 4, and the (+)
Amplifier 1 whose side input terminal is grounded and (-) of amplifier 1
a first switch 7 with one end connected to the side input terminal; a capacitor 3 with one end connected to the other end of the first switch 7; and an amplifier 1 with one end connected to the other end of the capacitor 3. a second switch 8 connected to the output terminal of the amplifier 1; a third switch 9 having one end connected to the connection point of the first switch 7 and the capacitor 3 and the other end connected to the output terminal of the amplifier 1; A fourth switch 10 has one end connected to the connection point of the second switch 8 and the capacitor 3 and the other end connected to the (-) side input terminal of the amplifier 1; A comparator 2 connected to the output terminal of the controller and whose (+) side input terminal is supplied with an analog voltage from an analog input terminal, and controls the first to fourth switches 7 to 10 based on the clock and the signal from the comparator 2. and a counter 6 connected to a digital output terminal for counting and outputting digital signals from the control circuit 5.

The conversion operation of such A'/D conversion circuit will be explained below.

FIG. 2 is an output voltage characteristic diagram of the amplifier in FIG. 1.

As shown in FIG. 2, the period of the clock is T.

When the output of the A/D conversion circuit is set to bits (k: integer), switch 7.8 is first turned off, switch 9.10 is turned on, and the reference voltage is applied until the counter 6 counts N (=2k) pieces. Integrate according to VREP (NT is the integration period). Next, turn on switch 7.8, switch 9
．． 10 is turned off and the same reference voltage V R,,
discharge (nT is the discharge period), and the comparator 2 determines when the output of the amplifier 1 crosses the input potential,
The control circuit 5 causes the counter 6 to stop. The value of the counter 6 counted down from the start of such discharge until the counter 6 stops becomes the output of the A/D conversion. That is, the value of the counter 6 is (N-n), where n represents the counted down value.

In short, this embodiment uses the switches 7 to 10 and controls them from the control circuit 5 to perform integration and discharge using the same reference voltage, and the analog input signal is directly used as the input signal of the comparator 2. Conversion errors are eliminated.

FIG. 3 is an A/D conversion circuit diagram showing a second embodiment of the present invention.

As shown in FIG. 3, like the first embodiment described above, this embodiment includes an amplifier 1, a comparator 2, and a capacitance of 3.17
It has a resistor 4, a control circuit 5, a counter 6, and switches 7-10, 11, 13-16.

This embodiment differs from the first embodiment described above in that it includes a switch 11 and a capacitor 17 for determining whether the input is positive or negative. The addition of an input reversible circuit consisting of switches 12 to 16 (possible due to the high input impedance) has the advantage of enabling bipolar A/D conversion.

〔Effect of the invention〕

As explained above, the A/D conversion circuit of the present invention has an amplifier, a comparator, a capacitor, a resistance control circuit, a counter, and a switch, and inputs an analog input directly to the comparator, and performs integration and calculation using the same reference voltage. By controlling the switch by the control circuit so that discharge can occur, the input impedance for the analog input can be made high, and the output impedance on the reference voltage side does not become a conversion error.

2 is an output voltage characteristic diagram of the amplifier in FIG. 1, FIG. 3 is an A/D conversion circuit diagram showing a second embodiment of the present invention, and FIG. 4 is an integration diagram showing a conventional example. FIG. 5 is an output voltage characteristic diagram of the amplifier in FIG. 4.

1...Amplifier, 2...Comparator, 3, 17...
-Capacitance, 4...Resistance, 5...Control circuit, 6...Counter, 7-16...Switch.

Claims (1)

[Claims] a resistor having one end connected to a reference voltage terminal; an amplifier having a (-) input terminal connected to the other end of the resistor and a (+) input terminal grounded; and a (-) input terminal of the amplifier. a first switch having one end connected to the first switch; a capacitor having one end connected to the other end of the first switch; and one end connected to the other end of the capacitor and the other end connected to the output terminal of the amplifier. a third switch whose one end is connected to the connection point of the first switch and the capacitor and whose other end is connected to the output terminal of the amplifier; a fourth switch connected to the connection point of the capacitor and having the other end connected to the (-) side input terminal of the amplifier; and a (-) side input terminal connected to the output terminal of the amplifier and the (+) side a comparator whose input terminal is supplied with an analog voltage from an analog input terminal; a control circuit that controls the first to fourth switches based on a clock and a signal from the comparator; and a control circuit connected to the digital output terminal. An A/D conversion circuit comprising: a counter for counting and outputting digital signals from the A/D conversion circuit.