CN87104460A

CN87104460A - Digital feedback double sampling high precision analog-digital converter

Info

Publication number: CN87104460A
Application number: CN87104460.9A
Authority: CN
Inventors: 费正生
Original assignee: HARBIN ELECTRICAL COLLEGE
Current assignee: HARBIN ELECTRICAL COLLEGE
Priority date: 1987-06-25
Filing date: 1987-06-25
Publication date: 1988-04-27
Also published as: CN1011012B

Abstract

Digital feedback double-sampling high-precision analog-to-digital converter, which is based on a general-precision double-integral A/D converter. In the second sampling, digital feedback is realized through control logic, programmable counters, and switch control. To replace the analog feedback realized by the high-precision D/A converter in the existing two-sample A/D converter. Compared with the existing two-sample A/D converter. Realizing digital feedback with digital circuits does not require complex and expensive high-precision D/A converters, high integration, low power consumption, high precision, strong applicability, and low price, and can be used to produce high-precision digital voltmeters.

Description

Digital feedback type double sampling high precision A/D converter

The present invention relates to an improvement of a double sampling analog-to-digital converter. The invention can be used to construct high-precision digital voltmeters.

The double sampling analog-to-digital converter is usually a hybrid high-precision converter comprising a low-precision a/D converter and a high-precision D/a converter. The analog-to-digital conversion of such a converter is achieved by a two-sampling process. At the first sampling, the analog quantity (voltage) is converted into a digital quantity by a low-precision a/D converter, which corresponds to a high number of bits. This high-bit digital quantity is then converted into an analog quantity by a high-precision D/a converter, which corresponds to the measured high-bit value. During the second sampling, the difference between the measured high-bit value fed back by the low-precision A/D converter and the measured high-bit value fed back by the high-precision D/A converter is converted by the low-precision A/D converter to obtain the measured low-bit number. The algebraic sum of the high and low digits represents the analog quantity to be measured.

In the double sampling analog-to-digital converter, a high precision D/a converter is a critical part, which determines the precision of the analog-to-digital converter.

An eight-bit 0.1ppm high-precision digital voltmeter published in No. 82 of 1974 in the journal of NIKKEI electroricnics, japan adopted a magnetic modulation type current comparator as a high-precision D/a converter. The D/A converter needs to use permalloy with high price, and has large volume and high cost.

The SM215 digital voltmeter of SELabs, uk, employs an inductive voltage divider as a high precision D/a converter, and also permalloy.

The TR-6567 digital voltmeter produced by the Japan martial arts and universities research adopts a resistor network as a high-precision D/A converter, has high requirements on a resistor and a switch, and has complex production process and high cost.

The three high-precision analog-digital converters with twice sampling have the common defects of large volume, difficult integration and high cost.

It is an object of the invention to improve the second sampling method of two samplings. The essence of the improvement lies in that the digital quantity obtained by the first sampling is directly fed back in the second sampling process, and is not converted into the analog quantity for feedback, so that the defects of the three high-precision A/D converters are overcome.

The invention is called digital feedback type double sampling high precision analog-digital converter because the high bit number obtained by the first sampling is directly fed back in a digital method in the second sampling process. The two-time sampling of the digital feedback type two-time sampling high-precision analog-digital converter is realized by a double-integral A/D converter with general precision. Of course, any A/D converter may be used for the first sampling as long as it can obtain the high number of bits to be measured.

In the first sampling, the ramp-up time T of the double-integrating A/D converter₁And a digital quantity N₁Accordingly, the down-slope time T_2hAnd a digital quantity H_2hAnd accordingly. Ramp over on second sampleIn the course of measuring E_xAccess time and digital mN₁Corresponding to the reference voltage E_sAccess time and digital mN_2hAnd correspondingly. This may be done by a counter N₁、N_2hAnd m, in fact when the up-down counter N is present₁And N_2hAfter reducing to 0, the original value N is restored again₁And N_2hRepeated m times, but in each set number of beats, N_2h≤N₁. The repetition of the number m can also be realized by an up-down counter. In the second sampling down-ramping process, E_xIs cut off, and the reference voltage for discharging can be reduced by n times to E_sAnd/n. Thus, in the second sampling, the low-order number N is obtained₂l。

Due to N₁And N_2hThe total number of the implantation is m times, and the reference voltage for the second sampling can be reduced to E_sN, so that the present invention can obtain a counting capacity of mnN_2h+N_2l. The digital feedback type double sampling analog-digital converter only uses a double integral A/D converter with general precision and some logic parts, but can obtain high-precision A/D conversion. The invention has low requirement on analog devices and simple logic, so the invention is easy to be made into large-scale integrated circuits.

The circuit diagram of the digital feedback double-sampling high-precision analog-to-digital converter shown in fig. 1 is explained as follows:

during the ramp-up of the first sample, 1 (± E) is measured_x) The switch 6 is closed to connect an integrator consisting of an integrating resistor 11, an integrating capacitor 17 and an amplifier 18, the capacitor 17 is charged, and the charging time T is₁ Programmable counter 30 may be clocked by control logic to count N₁The latter subtraction count is implemented. When N is present₁When decremented to 0, control logic 34, through switch control 29 and polarity detection and zero detection logic 28, switches 6 off and switches 7 on (when measured as-E)_xWhen measured as + E) or close switch 8 (when measured as + E)_xAt that time), the reference voltage is applied to discharge the capacitor 17, and the programmable counter 31 starts to count up from 0.

Once the charge in the capacitor 17 is discharged, the output of the zero detector 27 causes the polarity detection and zero detection logic 28 to operate, causing the programmable counter 31 to stop counting, and obtaining the count N_2hThis corresponds to the down-ramp time T_2h. N due to insufficient conversion accuracy of double-integral A/D_2hReaction substantially only of E_xThe high order number of (2). From the first sampling, the expression can be derived:

E_xh=E_s(T_2h)/(T₁) =E_s(N_2hτ₁)/(N₁τ₁) = E_s(N_2h)/(N₁) （1）

in the formula, E_xh-a high value of the measured voltage;

E_s-a reference voltage;

T₁-ramp-up time;

T_2h-down-slope time;

τ₁-the clock period used for the first sampling;

N₁，N_2hthe counting pulses in the

programmable counters

30 and 31, respectively.

After a rest time, the second sampling process is started. At this point, the programmable counter 30 places the number N₁Number N is reserved in 31_2hThe number m is set in the programmable counter 32 to ramp up the time T₁Repeat m times. During the upslope, the measured voltage 1 (assumed to be-E)_x) M beats are connected through a switch 6, and the switch conduction time of each beat is T₁. Meanwhile, reference voltage 2 (+ E)_s) M beats are also switched in by the switch 7, but the switch conduction time in each beat is T_2hTime T₁And T_2hIs a preset number N of pairs of

programmable counters

30 and 31₁And N_2hAnd performing subtraction counting. At this time, the output voltage of the integrator is:

<math><msub><mi>V </mi><mi>0</mi></msub><mi>= </mi><munderover><mi>Σ</mi><mi>j = 1</mi><mi>m</mi></munderover><mrow><mi>［</mi><mfrac><mrow><mi>1</mi></mrow><mrow><mi>C R</mi></mrow></mfrac><msubsup><mo>&Integral; </mo><mrow><msub><mi>( j - 1 ) T </mi><mi>1</mi></msub></mrow><mrow><msub><mi> j T </mi><mi>1</mi></msub></mrow></msubsup><mrow><msub><mi>E</mi><mi>X</mi></msub><mi>d t -</mi><mfrac><mrow><mi>1</mi></mrow><mrow><mi>CR</mi></mrow></mfrac><msubsup><mo>&Integral; </mo><mrow><msub><mi>( j - 1 ) T </mi><mi>1</mi></msub></mrow><mrow><msub><mi>( j - 1 ) T </mi><mi>1</mi></msub><mi>+ T </mi><msub><mi></mi><mi>2 h </mi></msub></mrow></msubsup><mrow><msub><mi>E </mi><mi>S</mi></msub><mi>d t</mi></mrow></mrow><mi>］</mi></mrow></math>

＝ (E_X)/(CR) mT₁- (E_S)/(CR) mT_2h（2）

when the ramp-up time reaches mT₁Switch 6 is turned off, and switch 7 is turned off when next tempo variable counter 31 is decremented to 0. According to V_oThe polarity detection logic 28 acts on the switch control 29 to turn on the switch 9 or 10 and switch in the corresponding reference voltage 4 (+ (Es)/(n)) or 5 (- (E)_s) And/n), and the control logic 34 starts counting by the programmable counter 33 until the output of the zero detector 27 activates the zero detection logic 28 to stop counting by the counter 33. At this time, the output voltage V of the integrator₀Is 0:

V^′ _o＝V_o+ 1/(CR)

<math><msubsup><mo>&Integral; </mo><mi>0</mi><mrow><msub><mi>T </mi><mi>2 L</mi></msub></mrow></msubsup></math>

（- (Es)/(n) ）dt

＝V_o- (Es)/(ncR) T_2L＝0 （3）

is represented by the formula (3)

V₀CR＝ (Es)/(n) T₂l （4）

Is represented by the formula (2)

V₀CR＝mE_xT₁-mE_sT_2h

Considering formula (1), the

V₀CR＝m〔E_xT₁-E_xhT₁〕＝mT₁〔E_x-E_xh〕（5）

As can be seen from equation (5), in m beats of the second sampling, the signal passes through mN_2hThe digital feedback of the voltage to be measured is realized by the low value E of the voltage to be measured_xlI.e.:

（E_x-E_xh）mT₁＝E_xlmT₁＝V_OCR （6）

is obtained by the two formulas (4) and (6)

E_xl＝ (E_S)/(mnT₁) T₂l （7）

The algebraic sum of the two sampling results is taken to obtain the measured voltage E_x：

E_x＝E_xh+E_xl＝E_s(T_2h)/(T₁) +E_s- (T_2l)/(mnT₁) （8）

In the second sampling, the clock period τ is ramped up₁May be a down-ramping clock period tau₂Q times of (1), so T₁＝N₁τ₁·T_2h＝N_{2h l}·T_2l＝N_2lτ₂Or T_2l＝N_2lτ₁And/q, and equation (8) can be written as:

E_x＝E_xh+E_xl＝ (E_S)/(mnqN₁) 〔mnqN_2h+N_2l〕（9）

as shown in formula (9), pair E_xThe first sampling can obtain high-order number N_2h. In the second sampling, the digital quantity N is passed_2hAnd the counting capacity can be expanded mnq times by reducing the reference voltage by n times and increasing the counting clock frequency by q times in the downward slope, thereby the conversion precision is remarkably improved without remarkably increasing the conversion time.

To increase the measurement speed, the first sampling can also be achieved by a low precision fast a/D35. The obtained digital quantity can also be used as N_2hDigital feedback is performed in the second sampling.

In fig. 1, the logic circuit 34 can be a single chip, a single board, or a logic circuit composed of discrete components, the counting

circuits

30, 31, 32, and 33 can be preset up-down counters, programmable counters, or count timing units in a single chip or a microcomputer, and the read-write memory 36 and the random access memory can also be included in the single chip or the single board microcomputer. Besides resistor and capacitor elements, most of analog circuits and logic circuits can be made into one or two large-scale integrated circuits.

Compared with the existing double-sampling A/D converter, the invention has the following advantages:

1. in the second sampling process of the two-time sampling, the digital feedback realized by a logic circuit is adopted, so that the precision is high and the cost is low;

2. on the basis of a common double-integration A/D converter, only a plurality of programmable counters and a single chip microcomputer or a microcomputer or other logic components are needed to be added, so that the integration level is high, and the volume is small;

3. the power consumption is low, and the reliability is high;

4. the parameters m, n and q are properly combined, the analog-digital conversion with various precisions and various speeds can be obtained on the same A/D converter, the use is flexible, and the application range is wide;

5. the function of the A/D converter can be greatly expanded by utilizing the combination of general precision double-integral A/D and low-precision quick A/D, so that the optimal compromise of measurement precision, measurement speed and cost is obtained;

6. and is easy to connect with a computer.

Drawings

Fig. 1 is a circuit diagram of a digital feedback double sampling high precision analog-to-digital converter.

1-input Voltage. + -. E_x2-reference voltage + E_s3-reference voltage-E_s4-reference voltage + E_sN, 5-reference voltage-E_sThe circuit comprises a voltage source, a voltage.

Fig. 2. a is a waveform diagram of the integrator output during two sampling.

Fig. 2. B is a graph of the input voltage and reference voltage waveforms during two samplings.

Claims

1. The invention relates to a double-integral A/D converter composed of an integrator, a comparator, polarity detection and zero detection logic and control logic, and a digital feedback type double-sampling high-precision A/D converter composed of digital feedback control logic, a programmable counter and switch control.

2. A digital feedback double sampling high accuracy analog to digital converter as claimed in claim. The method for realizing high precision comprises the following steps: in the second sampling process, all or part of high-order digits of the digital quantity obtained by the first sampling are preset in a programmable counter N_2hAs a digital feedback quantity, the time for discharging the integrator by the reference voltage is controlled in a manner of subtraction, and another programmable counter N is used₁After a certain digital quantity is preset, the time for charging the integrator by the measurement is calculated in a subtraction counting mode, and because the digital feedback is carried out for m beats, the reference voltage used in the second sampling down-slope is reduced by N times and the clock frequency used is improved by q times, so that the digital number N obtained by the first sampling_2hMnq times, thereby improving the accuracy of the A/D converter.

3. The digital feedback double sampling high precision analog-to-digital converter as claimed in claim 1, wherein the general precision double integral A/D converter is available in the off-the-shelf market, such as large scale integrated circuit double integral A/D converter or general precision digital voltmeter.

4. The digital feedback double sampling high accuracy analog-to-digital converter as set forth in claim 1, wherein the control logic is formed by a single chip, a single board computer, a microcomputer system, a computer system, a large scale integrated circuit or discrete components.

5. Digital feedback double sampling high precision analog-to-digital converter as claimed in claim 1 having a programmable counter N₁，N_2hM and N_2lIt can also be formed by presetting reversible counter or counting unit in single chip, single board computer, microcomputer system or computer system, or can be made into special large scale integrated circuit.

6. The digital feedback double sampling high precision analog-to-digital converter as claimed in claim 1, wherein the first sampling can be performed by other a/D converters than the double integrating a/D converter.

7. The digital feedback double sampling high accuracy analog-to-digital converter as claimed in claim 1, wherein the read only memory and the random access memory are also included in a single chip, a single board computer, a microcomputer system, or a computer system.

8. The digital feedback double-sampling high-precision analog-to-digital converter according to claim 1, wherein all or most of the analog circuit and the digital circuit are integrated in one or more LSI circuits.

9. The digital feedback double sampling high precision analog-to-digital converter according to claim 1 and the digital feedback method according to claim 2 can be based on different combinations of parameters m, n and q to achieve the required conversion precision and conversion speed.