experimental evaluation of null method and dc-ac...

Proceedings of International Conference on Technology and Social Science 2019 (ICTSS 2019)

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Experimental Evaluation of Null Method and DC-AC Conversion

for Operational Amplifier Testing Shogo Katayama1, a, Riho Aoki1,b, Yuto Sasaki1,c, Kosuke Machida1,d

Takayuki Nakatani1,e, Jianlong Wang1,f, Anna Kuwana1,g

Kazumi Hatayama1,h, Haruo Kobayashi1,i, Keno Sato2,j,

Takashi Ishida2,k, Toshiyuki Okamoto2,l, Tamotsu Ichikawa2,m

1Division of Electronics and Informatics, Faculty of Science and Technology, Gunma University

1-5-1 Tenjin-cho Kiryu, Gunma, Japan 376-8515

2ROHM Co., Ltd., 2-4-8 Shin Yokohama, Kohoku-ku, Yokohama 222-0033, Japan

a<[email protected]>,b<[email protected]>, c<[email protected]>

d<[email protected]> , e<[email protected]>

f<[email protected]>, g<[email protected]>, h<[email protected]>

i<[email protected]>, j<[email protected]>, k<[email protected]>

l<[email protected]>, m<[email protected]>

Keywords: high precision operational amplifier, offset voltage, Null method, DC-AC conversion, low level DC voltage measurement, thermo-electromotive force

Abstract. This paper examines the offset voltage measurement for the precise operational amplifier at the μV-order level with the Null method and the DC-AC conversion method. We have implemented their prototype circuits and evaluated their performance experimentally.

1. Introduction

In recent years, the demand for various types of sensors is increasing as the IoT technology prevails. Analog circuits such as high-precision operational amplifiers and ADCs for various sensors are key components there. Their high-quality and low-cost testing at mass production shipping stage is required for cost effective and reliable IoT systems. The precision operational amplifier has a differential input, a single ended output and extremely high gain. In particular in case that its open-loop gain is �� or more, it is very difficult to suppress minute voltage errors. [1-5]

The operational amplifier precision measurement/characterization at the laboratory level usually uses a Null method [1] or a precision digital voltmeter. On the other hand, its mass production testing uses Automated Test Equipment (ATE), which requires short time for low cost and has system noise issues. Notice that the measurement/characterization and the testing are similar, but different technologies. In our previous paper [6], we focused on the mass production test and we studied the DC-AC conversion technique that combines chopper and FFT techniques to overcome these problems; the DC-AC conversion technique enables multi-channel testing for operational amplifiers in parallel, which equivalently reduces the testing time for one operational amplifier under test. Also it enables precision testing at �V-order level by removing a great deal of system noises in ATE systems.

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In this paper, we examine the precise DC voltage measurements with the Null method and the DC-AC conversion method and discuss their comparison. Also experimental verification is shown.

2. Measurement Technique for Operational Amplifier Characteristics

2.1 Null Method

The Null method is widely used to measure many parameters of the operational amplifier characteristics using the Null circuit in Fig. 1. [1] The Null circuit consists of an operational amplifier as a device under test (DUT) with power supply pins (+VP, -VP), an auxiliary operational amplifier for an integrator and several switches (S1, S2, S3, S4) to set the measurement items, as well as resistors and capacitors. This circuit forms a stable loop with a very high DC open loop gain, and the DC open loop gain of the auxiliary operational amplifier should be more than 10�, though it need not be higher than the DUT performance.

Fig.1. Null circuit [1]

3. Experiment Verification

3.1 Devices Under Test

A precision operational amplifier (AD8571) with auto-zero offset voltage canceling circuit is used as a low offset operational amplifier under test. Measurements of its input offset voltage using the Null method and the DC-AC conversion method were performed. Four operational amplifiers (AD8571) are measured as samples, which are referred No.1, No.2, No.3, No.4 hereafter.

3.2 Null Circuit Configuration

We have implemented two types of Null circuits, named A, B, with which we have measured the input offset voltages of four operational amplifiers (AD8751). Fig. 2 shows a circuit diagram of the Null circuit, and Fig. 3 (a), (b) show photographs of the Null circuits. Each Null circuit A, B uses an IC socket to share the DUT and the auxiliary operational amplifier (LF356), and also resistors with 1% grade metal film.


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(a) Null circuit A. (b) Null circuit B.

Fig.2. Null circuit diagram of our prototype Fig.3. Prototype Null circuits.

3.3 Input Offset Voltage Measurement Results by the Null Method

We have measured the output of the Null circuit with a digital voltmeter and averages 20 times measured data for each operational amplifier. The gain of the Null circuit is 1,000, and hence the division of the output voltage by 1,000 was performed in order to convert the output to the input-referred offset voltage. The results are shown in Fig. 4, and each solid line indicates the average of the measured values.

(a) Null circuit A (b) Null circuit B

Fig.4. Measurement result of Null method

The measured offset voltages �� have a difference by about 1μV between Null circuits A , B; this

is probably due to the difference of the thermo-electromotive force effects in the DUT input unit in the Null circuits A and B.

3.4 DC-AC Conversion Circuit

Fig. 5 shows a circuit diagram of the DC-AC conversion and spectrum measurements [6], and the DC-AC converter is composed of a switching circuit using a TC4053, an instrumentation amplifier and an AC amplifier. The outputs of the DC-AC converter are acquired by the USB-6003, and their FFT spectrum measurements with the DC-AC conversion clock frequency using LabVIEW programming.

As an initial stage calibration, a DC voltage of 10V through an attenuator of 120dB is applied to the input of the DC-AC converter circuit; in this case, J1 is switched to the CAL side in Fig. 5 and FFT spectrum measurement results are normalized to 10μV. The DC-AC converter positive input is


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connected to the DUT output pin and the other negative input is to GND for a differential DC-AC

converter. Also the input resistance of the DUT is set to 100Ω by ��. Then an input offset voltage is obtained with dividing the FFT spectrum result by gain of the DUT.

Fig.5. DC-AC conversion circuit

3.5 DC-AC Conversion Spectrum Measurement Result

We have measured the sample No.1 under the following conditions: sampling frequency of 100kHz, sampling times of 10,000, DUT gain of 1,000 time, a 1kHz rectangular wave clock for DC-AC conversion. Measured FFT spectrum is obtained with the LabVIEW program using 10,000 samples and an average of 100 times. In addition, the DC-AC conversion output was connected to the oscilloscope input, and we measured the spectrum using its FFT function with 64-times averaging in time domain.

Spectrum measured in LabVIEW program is shown in Fig. 6, and the spectrum measured by the oscilloscope FFT function is in Fig. 7.

Fig.6. Spectrum of the DC-AC converter output (LabVIEW)


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Fig.7. Spectrum of the DC-AC converter output (Oscilloscope) The resulting spectrum noise floor by LabVIEW program is relatively large and the spectrum at 1kHz could not be measured, whereas the noise floor obtained by the oscilloscope FFT function is relatively low so that the spectrum could be measured.

3.6 Improvement by Averaging Technique

We have implemented the synchronization averaging function in time domain using LabVIEW measurement program; its averaging number is 10, the DC-AC conversion clock frequency is 1 kHz, DUT gain is 10, 100 and 1,000. Also for each sample No.1, … , No.4, 100 times measurement is performed. Fig. 8 shows the spectrum obtained by the above averaging technique. We see that the floor noise is reduced by the synchronization averaging in time domain.

Fig.8. Spectrum of DC-AC converter output (Improved program)


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Fig. 9 shows the input offset voltage for each DUT and each gain, and there the solid line shows the averaged value. We see that the measured offset voltage value is degreased in accordance with increase of the gain. Fig. 10 shows the measured input offset voltage of each DUT for its gain of 1,000.

(a) DUT No.1 (b) DUT No.2

(c) DUT No.3 (d) DUT No.4

Fig.9. Measurement result of DC-AC & FFT

Fig.10. Operational amplifier input offset voltage measurement results by DC-AC conversion.


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�� measurement results by the Null circuit A and the DC-AC conversion are shown in Table 1.

Table 1. Operational amplifier input offset voltage (Vos) measurement result comparison

Input offset voltage �� [μV] Sample Null A DC-AC

No.1 0.43 0.27 No.2 0.62 0.36 No.3 0.45 0.23 No.4 0.92 0.59

�� measurement results by the Null circuit A and the DC-AC conversion show a similar trend for

all four DUT samples. Each measured �� by the DC-AC conversion is lower by 0.2μV - 0.3μV than that by the Null circuit A for each sample; this is estimated to be due to the difference in thermal electromotive force effects of their substrates.

4. Conclusion

In this paper, experimental comparison for high-precision DC voltage (the input offset of the operational amplifier) measurement using the Null measurement method and the DC-AC conversion method has been discussed. Large measurement result difference (error) may be caused depending on the prototype Null circuit; this is probably due to the effect of thermal electromotive force.

The DC-AC conversion spectrum measurement has reduced the noise effects by synchronous averaging in time domain significantly. This is probably due to the noise by an auto zero offset voltage cancellation circuit of AD8571 as a DUT in the range from 2 kHz to 4 kHz.

In our experiment this time, the Null circuit measurement results are influenced by the thermal electromotive force and the DC-AC conversion measurement results are by the noise. As future projects, we will study the component layout technique to reduce the effects of the thermal electromotive force in the Null circuit as well as the switching frequency and averaging method for noise and measurement time reduction in the DC-AC conversion method.

References

[1] J. M. Bryant, “Simple Op Amp Measurements”, Analog Dialogue, vol. 45. pp 21-23, 2011.

[2] Op Amp Applications Handbook, Analog Devices, 2004.

[3] Kumen Blake, “Op Amp Precision Design: PCB Layout Techniques”, Microchip Technology Inc., Tech. Rep. AN1258, 2009.

[4] R. Dopkin, Analog Circuit Design, Linear Technology, 2013.

[5] G. Robert, F. Taenzler, M. Burns, An introduction to mixed-signal IC test & measurement, 2nd Edition, Oxford University Press, 2012.

[6] K. Machida, Y. Sasaki, T. Nakatani, K. Sato, T. Ishida, T. Okamoto, T. Ichikawa, J. Wang, A. Kuwana, K. Hatayama, H. Kobayashi, “Low-level DC voltage measurement technique based on DC-AC conversion”, ECT-18-88, IEEJ Electronic Circuit Workshop, (Tokyo) Jan. 2018.

experimental evaluation of null method and dc-ac...

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