MULTICHANNEL ANALOG-TO-DIGITAL CONVERTER FOR BIPOLAR VOLTAGE

V. L. Ryabukhin

(Presented by the scientific seminar of the Department of Mathematical and Computing Instruments and Devices)

In automatic control and regulation systems, devices are often used for converting a voltage from continuous form into discrete form.

Usually, in order to simplify the converter circuit and improve the accuracy of its operation, information sensors are supplied with unipolar voltage. This article considers elements of an analog-to-digital converter for bipolar voltage.

The converter is built on the principle of comparing the input voltage with a linearly varying voltage. As the generator of bipolar sawtooth voltage in such converters, either an integrator based on a direct-current amplifier is used, or a unipolar-signal generator with an inverter and additional logic circuits, which reduces the reliability of the device when operating over a wide temperature range.

In the converter developed, a conventional sawtooth-voltage generator with a compensating emf is used, producing a bipolar sawtooth voltage varying in the range $\pm 5$ V. Its distinctive feature is that two triodes, connected in opposition in a compensation circuit, are used as the switch ($T_3$—$T_4$ in Fig. 1).

The control high-frequency voltage, applied through a full-wave rectifier to the base–collector circuit of the triodes, is taken from the output winding of a blocking-oscillator transformer ($T_1$—$T_2$) isolated from “ground,” operating at a frequency of 300–500 kHz and controlled through the triode $T_2$ by the trigger of the sawtooth-voltage generator. A switch with compensation connection of the triodes makes it possible to commutate a voltage of either polarity, has a stable transfer coefficient in the open state and a high reverse resistance in the closed state. Tests have shown that, with an overall conversion error of 3%, these switches do not affect the stability of the sawtooth-voltage generator over the temperature range $-50$–$+65^\circ$C. At the output of the generator a composite emitter follower is connected, possessing a high input resistance and a low output resistance. In the initial state, when the switch is open, at the generator output there is a voltage of $+5.2$ V, taken from the divider $R_3R_4$.

In the converter, a separate comparator is used for each channel and one comparator common to all channels, which produces a signal when the sawtooth voltage passes through zero. The order of operation of the selected channel comparator and the zero-level comparator determines the sign of the input voltage. The input section of the comparator \((T_7 \div T_9)\) is an unbalanced dc differential amplifier, whose emitter load, in order to stabilize sensitivity over the entire range of variation of the input voltage, is an emitter follower. In the initial state triode \(T_7\) is open and triode \(T_8\) is closed, since the input voltage applied to the base of triode \(T_7\) varies within the range \(\pm 5\) V, while the base of triode \(T_8\) receives a voltage from the output of the SFG equal to \(+5.2\) V. At the instant when the input and sawtooth voltages are compared, triode \(T_8\) opens, and the positive voltage step, after passing through emitter follower \(T_{10}\) and the differentiating circuit \(C_5R_{12}\), is applied to the base of amplifier \(T_{11}\). From the collector of triode \(T_{11}\) a negative pulse with an amplitude of 12 V is taken, which is applied to the channel-selection gate \((R_{13}, D_3, \mathrm{Tr}.\ 2)\). To the second input of the gate, through resistor \(R_{13}\), a voltage is applied from the channel decoder. From the output of transformer Tr. 2 the pulse of the required channel is applied to the terminal pulse amplifier.

Investigation of the converter noise was carried out by the method described in [1]. However, the principle described in that work for synchronizing the start of conversion with the pulses of the counting-pulse generator, used to eliminate error due to the uncertainty of the start-pulse position \(\Delta t_1\) [2], cannot be implemented when the initial portion of the sawtooth voltage, which usually has large nonlinearity, is not used, or when a bipolar sawtooth voltage is used. In studying the noise of the converter in such circuits, it is convenient to connect to the input of the main counter a frequency divider by 4 or 8, as a result of which the error due to the uncertainty \(\Delta t_1\) can be reduced by a factor of 8. It is also possible to use, as the counting-pulse generator, a shock-excitation generator. The circuit of such a generator, which is a blocking oscillator with stabilization of the pulse repetition frequency by means of a delay line, is assembled on triode \(T_{12}\).

Depending on the polarity of the input voltage, the generator is started by a pulse from the channel comparator or from the zero-level comparator. Simultaneously with this, a negative voltage step is applied to the base of triode \(T_{13}\) from the output of the trigger controlling the admission of pulses to the counter input, as a result of which the triode opens and the delay line is short-circuited at the end. The generator produces pulses with repetition period \(\tau = 2t_{\text{delay}} = 1.5\ \mu\text{s}\). When the trigger returns to the initial state, the delay line becomes loaded by the characteristic impedance and generation ceases.

Tests of the converter circuit showed that the converter’s intrinsic noise is considerably less than the quantization noise, and that under laboratory conditions the number of digits in the counter can be increased to 8.

Technical characteristics of the converter:

Input-voltage variation range — \(\pm 5\) V.
Conversion error — 3%.
Operating-temperature range — \(40 \div 50^\circ\text{C}\).
Conversion time for one channel — 150 \(\mu\text{s}\).
Number of channels — 8.

References

1. G. A. Danilovich, A. Yanyshek. A voltage-to-digital-code converter, in the collection of articles Problems of Pulse Technology and Electronic Computing Devices. Oborongiz, 1960.

2. E. I. Gitis. Information Converters for Electronic Digital Computing Devices. Gosenergoizdat, 1961.