IEEE Transactions on Nuclear Science, Vol. 33, No. 1, February 1986

PRECISION DIFFERENTIAL VOLTAGE PROBE FOR TRANSIENT RECORDER

Joe Ting, Dan Amidei, Myron Campbell, Bill Nakagawa, George Redlinger, Harold Sanders, Scott SleeperThe Enrico Fermi Institute, The University of Chicago, Chicagq, IL 60637

Abstract

A two input differential precision voltage probewas designed and built to accurately measure (to 10bits) high impedance analog signals in a FASTBUS en-vironment, where only noisy ground reference is avail-able. The output settles to 0.1% in less than 300 nsand drives 50Q to ±2V. It is an ideal probe to matchthe performance of digital oscilloscopes.

Introduction

There has been a continuous trend to get fasterand more precise measurements of any phenomena. So itis in the determination of the dc and ac character-istics of fast pulses. While the speed challenge hasbeen met and conquered, the precision challenge leftmuch to be desired for the lack of suitable measure-ment instruments. The advent of digital scopes isbeginning to fill in this gap. While this kind ofinstrument brings about a revolution in its field,most of the available high impedance probes are stillthe old workhorses of the analog oscilloscopes. Inmany instances, as the systems to be measured becomemore complex, these probes are no longer adequate,because either they alter the system by introducingundesired ground loops or because the ground referencesavailable are too noisy.) A solution is to float thescopes. However it is potentially dangerous due toshocks and very often it does not provide enough commonmode rejection. We were faced with this problem whenwe tried to measure settling times of analog signalsamidst a jungle of ECL chips, on the FASTBUS boards forthe CDF trigger.

A good digital oscilloscope probe must have thefollowing characteristics:

1) High impedance, both dc and ac.2) High precision, linearity, and stability: to

better than 0.5%.3) Truly floating input.4) Speed comparable to the analog input of the

scope, with particular attention to attainfast settling time.

5) Be able to drive 50Q in order to avoid theneed to compensate for the input capacitanceof the scope.

6) Versatile and easy use.

Following these guidelines, a probe was designed,built and calibrated.

The architecture chosen is that of an instrumen-tation amplifier, because of its inherent high commonmode capability. The probe consists of two assembliesconnected by cables. An input box which contains twoidentical input buffers, one for inverting and anotherfor non-inverting channels; and a differential receiverbox which can drive ±2V into 50Q.

For the sake of versatility, two test clips withshort lengths of wires couple the points to be mea-sured to the probe inputs. There is no third localground connection which makes the probe very easy touse. The absence of a local ground is usually a causeof oscillations; this is circumvented by the additionof a passive network at the probe input. Since theinputs are truly floating, differential measurementsbetween any two points can be made, as long as thecommon mode is not exceeded. For single ended measure-

ments one input is attached to local ground.Because of the precision required, the high

input impedance buffers in the input box must be feed-back amplifiers. Open loop FET source followers orbipolar emitter followers are not accurate enough.Therefore, by choice, each input buffer is a JFET 100%feedback op-amp, made up of transistor arrays. Thetwo buffers share two common transistor packages sothat temperature drifts are tracked and cancelled inthe receiver. The dual JFET input transistors aremounted on the same heat sink. The buffer circuitshave only one stage of voltage gain, so that the para-sitic capacitances are reduced and consequently, thesettling time is minimized. The network that preventsoscillations at the probe input is a small toroidtransformer that couples the two inputs together.Since only seven turns are used in the windings, theeffect of input inductance was not measurable. Thetwo outputs of the probe drive a balanced twistedshielded pair of cable that is connected to the re-ceiver box on the other end. Another cable providespower to the probe. See Fig. 1.

For convenience, the receiver box (see Fig. 2)was built into a CAMAC module. It has four channels,so that four probes can be used simultaneously. Eachreceiver performs several functions:

1) It combines the inverting and the non-inverting signals into a single ended one.

2) It has frequency shaping adjustments.3) It has a three position signal output

attenuator switch.4) It has an offset adjustment pot.5) It drives ±2V into 50Q.

Special attention was paid to achieve a probedesign that does not produce overshoots and rinqing.

The components used were carefully selected fortheir precision, stability and speed. Even signalcables had to be tested and chosen for low dieletricabsorption. Dearborn #942802 was used. Wheneverprecision and speed were conflicting requirements, theprecision was incorporated at the expense of speed.Standard high frequency circuit layout techniques wereused in the design and layout of the pc boards.

Calibration

It was difficult to select a suitable absolutesignal reference source as the waveshape calibrationstandard. After several tries, a Tektronix model 110mercury relay pulse generator was chosen. See Fig. 3.

Achieved Electrical Characteristics

Input attenuationInnut impedance

Frequency ResponseSettling time to 0.1%Ri setimeMaximum InDut Range

CMRRNoise (inputs shorted)

:Xl or X10.1IMQ in parallel with l1nFfor Xl innut or 1MQ inparallel with 6nF for theX10 input.:Better than lOMHz.:250 ns.*40ns.:+2V for Xl or ±20V for X10inputs:Better than 50dB.:900 pVrms.

0018-9499/86/0200-0907$01.00©1986 IEEE

907

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Fig. 2

908

909

Calibration Waveforms

1 V/div Pulse generator output

500 mV/div Calibrator box outDut

50 mV Expanded view of waveform above

Fig. 3

100 ns/div

PRECISION DIFFERENTIAL PROBE CALIBRATION DIAGRAM UNIVERSITY OF -CHC01O

Fig. 4

1 V/div

100 mV/div

10 mV/div

Probe response

Notice the same

Fig. 5

for different levels of fast inDut pulses

ri setime

100 ns/div

910

Conclusions

This probe was used extensively during theprototype evaluation phase of the RAll and CAS boardsof the CDF Trigger.

Originally a LeCroy 2261 Image Chamber Analyserwas used as a 10 bit transient recorder, but wefinally settled for a 8 bit digital scope instead,because the 2261 needed too much software development.The other reason is because a bulky LSI-ll computerwas part of the system, which makes it hardly portable.

Wlore imorovements can certainly be made on thisprobe. For instance, more speed without compromisingthe accuracy would be useful. A bootstrap or otherfeatures to decrease input capacitance would bedesirable.

The input bu-Ffer box and the CAMAC quad receiver

Using the probe to measure a FASTBUS card An inside view of the orobe

330 mV

320 s.. Ls/div Expanded view

Measuring an op-amD on a FASTBUS card using the Drobe.The recorder used was a LeCROY 2261 Image Chamber Analyser

I s mV

0000..