ApplicationNote Recalibrating the AVS-47B

AVS-47B

Revised: 2010-10-21 recalibrating AVS47B.indd RV-Elektroniikka Oy PICOWATT Page 1

RV-Elektroniikka Oy PICOWATTVeromiehentie 14FI-01510 VANTAA, Finlandphone +358 50 337 5192e-mail: reijo.voutilainen@picowatt.fiInternet: www.picowatt.fi

WHY AND WHEN TO RECALIBRATE

The AVS-47B consists of two parts, the 19-inch main unit and the separate preamplifier box. Both have serial numbers: the number of the main unit can be found on the rear panel and also inside the unit. The preamplifier’s complete number is inside the box, but the first digits of the number are also visible on the front panel of the box. All new AVS-47B instruments have equal serial numbers on these two units, they have been calibrated as a single entity, and it is very important that the preamp+main unit pairs are kept together. If they are mixed, the calibration is lost, and scale factors of various measuring ranges may differ by some tenths of a percent, in the worst case. The simple “self-calibration” that can be done from the front panel affects the overall offset and scale factor of the in-strument, but it does not reveal variations between ranges.

The preamplifier box contains, in addition to two al-most identical preamplifier channels, six range-determin-ing resistors that pass the excitation current to the sensor. They have been placed in the preamp box in order to keep the sensitive wires to the sensors as short as possible. These wire-wound resistors have extremely good tem-perature and long time stability, but their initial tolerance is only 0.1%. One cannot trim the range resistors directly: the trimmers would generate unpredictable contact noise, and therefore trimming must be done in an active circuit, where the contact noise can be eliminated. Because of space limitations inside the preamplifier, the adjustment circuitry is located inside the main unit. This explains, why the two boxes must be always handled as a pair.

It can happen, however, that the boxes are mixed by accident and that the original order can no longer be restored. Then various ranges can differ by as much as some tenths of a percent. Some repairs can also change the calibration of either box. Experience has shown that many bridges have been sent -and unfortunately also will be sent- to us for repair without the preamplifier. In all these cases, recalibration is necessary in order to restore the accuracy of the AVS-47B. Until now, calibration agains external standards at the factory has been the only

possibility. The purpose of this application note is to en-able an experienced user to check and correct the calibra-tion to a decent level of accuracy. The process requires a small calibrator board but no expensive resistor standards (and therefore the result will not be quite as good as the factory calibration). Calibrating the deviation reference and the difference amplifier is not necessary if you do not really use the difference output. Similarly, you can leave the excitations as they are - at least the three lowest ones.

NO SPECIAL EQUIPMENT IS REQUIRED

One needs an AVS47 calibrator board. Such a board is obtained from us in mail free of charge, if recalibration or at least checking the ranges seems to be necessary. The board has a set of cheap 50ppm/°C resistors with 1% initial tolerance. Therefore, the board is accompanied by a table that lists more accurately measured values, which you will use for calibration. The board is used for:

1. Setting scale factors for ranges from 2 W to 2 M W. 2. Verifying the condition of multiplexer relays

With the calibrator board and one crosshead and one small flathead screwdriver, you can recalibrate the AVS-47B so that it shows correct values on the display and sends correct readings to the computer interface. If you are using an external voltmeter connected to the ANA-LOG output, the process includes also calibrating the analog output with your existing meter. Use a coaxial cable for connecting the DVM in order to reduce the risk of interference pick-up.

The white laquer that has been used for securing the trim-mers does not prevent re-adjustment.

ApplicationNote Recalibrating the AVS-47B

AVS-47B

Revised: 2010-10-21 recalibrating AVS47B.indd RV-Elektroniikka Oy PICOWATT Page 2

1. START WITH SELF-CALIBRATION

With the preamplifier connected to the main unit, open the top cover. Use a small screwdriver to find the position, where the cover can be lifted up, as shown by the photo below.

Switch on the power and let the unit warm up without cover for at least half an hour.

Select mode=CAL, range=20 kW , excitation=3mV. Set the display to 0.0100 with the front panel ZERO trimmer. Skip the box below if you do not measure the ANALOG output.

If you have the optional external voltmeter, set now mode to ZERO. The DVM should show about 0.3mV, as there is a small shift between the offsets of the analog output and the panel display. If your DVM has a nulling feature, you shall null it now. The panel meter may show a reading from -1 to +1. The A/D converter IC has different modes for positive and negative inputs, and therefore it does not meas-ure zero very well. You can adjust either the external DVM or the display to exact zero, but not both.

Select mode=CAL, range=200 W and excitation=3mV.

If you have the optional external meter, set it to 1.0000V using the front panel SCALE trimmer. Then use trimmer R107 (A/D scale) on the “E” circuit board, if necessary, to set the front panel display to 100.00.

If an external DVM is not connected, use the front panel SCALE trimmer to set the display to 100.00.

These adjustments set the overall offset and gain on the 3mV excitation range, but all ranges and all other exci-tations are still intact. Possible drift due to temperature

changes or aging will not cause significant changes between ranges, whereas changing the preamplifier will. Drifts in overall offset and gain are easily nulled by using the ZERO and SCALE adjustments.

2. RANGES

Connect the calibrator board to the AVS-47B preampli-fier as shown by the next photo. Select mode=MEAS, 2W range, 3mV excitation and input channel 0. Look the cor-rect value for the 1W resistor in channel 0 from the table

that accompanies the calibrator board and use R122 on the “D” circuit board to adjust for correct reading.

Select 20W range and input channel 1. Use trimmer R121 to adjust the reading.

Continue up to the 2 MW range and channel 6. This completes adjustment of the ranges.

3. THE MULTIPLEXER

The multiplexer board inside the preamplifier box con-tains fourteen 2-contact reed relays, 2 relays for each of the 8 sensor channels. Relays are electromechanical devices with very long but still finite lifetimes. In spite of the possible increase in contact resistance, the AVS-47B will measure correctly, because the 4-wire sensor connec-tion eliminates the effect of lead resistances. However, if the contacts become unreliable or refuse to make a proper contact at all, then the bridge will become noisy or show crazy and random readings, or it will indicate “high lead resistance” error. Checking the multiplexer relays with the aid of the calibrator board can help either in confirming or eliminating this fault possibility.

Insert the calibrator board as shown by the next photo. The board has eight ~1W resistors, one for each of the 8

ApplicationNote Recalibrating the AVS-47B

AVS-47B

Revised: 2010-10-21 recalibrating AVS47B.indd RV-Elektroniikka Oy PICOWATT Page 3

input channels. Use the 2W range and 3mV excitation. Select each channel in turn and compare the reading with the table that came with the calibrator board. Then set the DISPLAY switch to EXC VOLT position and go through the channels again. The display should be something like 35-45 for good relays. The resolution of this method is not sensitive enough for detecting a small increase in contact resistance, but if the increase is of the order of 5 to 10 ohms in a current lead, the reading should be significantly higher. Voltage relays are not at all so critical, because they do not carry any current.

Close the top cover. After an hour or so, check the self-calibration again.

4. EXCITATIONS (optional)

Excitation ranges are by far not as likely to change as the ranges, because all excitation circuitry is located inside the main unit. The 3mV excitation adjustment (R125 on the “D” board) must not be touched, because this was in-cluded in the previous procedure of calibrating the ranges. The next highest excitations from 1mV to 100mV are still easy to check (adjust trimmers R126, R127 and R128, re-spectively, if needed) because the readings are still quite stable and noiseless.

Excitations from 30mV down to 3mV are more difficult because of the random noise in the readings. The only accurate way to calibrate them is to use averaging, which requires interfacing with computer. Quite many readings are required in order to get useful averages: at least 30 for 30mV, 100 for 10mV and at least 300 for 3mV. Because the noise maximums and minimums are spaced sym-metrically around the mean value, one can alternatively use some time observing such readings. If they seem to be significantly unsymmetric, then recalibration of these excitations may be needed. Please note that the differ-ence between max and min on the 3mV excitation range can be as much as 50 digits, whereas a drift of 10 digits is

unlikely during the lifetime of the bridge. We recommend that you change the calibration of the lowest ranges only, if you estimate that the mean value differs from correct value by an amount that is comparable to noise.

5. THE DEVIATION REFERENCE (optional)

The deviation reference circuitry is rather stable and re-adjustment is recommended only if you are going to calibrate also the difference amplifier. The reference volt-age is subtracted from the analog output voltage in a dif-ferential amplifier, and the difference is available at a rear panel BNC connector. It can also be directed to the A/D converter and monitored using the DR display. The dig-ital-to-analog converter that forms the reference voltage has 12 bits (0..4095) whereas the front panel display (and the A/D converter) have 4 1/2 digits (0..19999, approxi-mately 14 bits). Therefore a hardware circuit divides the displayed reading by 5 before it is fed to the D/A convert-er. Note this when calibrating the reference: the reference has steps of five display units plus the natural differential and integral linearity errors of a D/A converter. The refer-ence voltage source is checked and adjusted as follows:

Select 200W range, 3mV excitation and ZERO mode. Wait until the R display has settled to zero (+/- 1). Lift the SET REF button (not too fast). Switch the DISPLAY to REFERENCE. Adjust the reading to zero by trimmer R401 if necessary.

Select CAL mode. Wait until the R display has settled to 100.00 +/- 2 digits. Lift the SET REF button. Switch the display to REFERENCE and adjust to 100.00 by trimmer R402 if necessary.

6. THE DIFFERENCE AMPLIFIER (optional)

The DIFFERENCE voltage output dates back to times when graph recorders were still commonplace in labo-ratories. With such a recorder, one could use a sensitive scale only if the graph was centered, or nulled, before the measurement. Nulling the starting point before record-ing a resistance change was the main purpose of the DR output. Another possible use of this output is to extend the range of the AVS-47B from 20000 to 30000 in an “emergency case” by setting the reference to 10000 (the DR display goes then from -10000 to 20000 correspond-ing to input resistance from 0 to e.g. 30 kW). It is also possible to increase the digital resolution of the bridge by selecting the 10xDR mode (this mode should be used only for measuring small changes). Whereas the ANALOG output is well calibrated and the most stable output of the

ApplicationNote Recalibrating the AVS-47B

AVS-47B

Revised: 2010-10-21 recalibrating AVS47B.indd RV-Elektroniikka Oy PICOWATT Page 4

bridge, the DIFFERENCE output has uncertainties due to the 12-bit reference and the nonideality of the analog differential amplifier. It must be considered only as a less important feature of the AVS-47B. We recommend that the DR modes are not recalibrated unless they are really in use. The trimmers for calibrating DR modes are located under the small capacitance compensation circuit board. If you find them too difficult to reach, you can unscrew the board. Do not remove the cables, and see carefully

that the board does not touch any parts of the bridge. Use some plastic or paper to prevent contacts.

The bridge OFFSET and SCALE, and also the reference must have been checked earlier.

Select ZERO mode, 20kW range, 3mV excitation and R display. Lift the SET REF switch in order to store the reading 0.0000 as reference. You should verify pro-

grammed reference voltages by visiting the REFERENCE display. If the display is something strange, repeat the programming, but keep the switch lifted for a little longer time. The bridge is now measuring 0W and the reference is 0. Lift the DR/10xDR switch to the 10xDR position. The difference should be 0. Use trimmer R203 for adjustment, if needed. Set the DR/10xDR switch to DR and verify that the offset is still 0. These two gains have one single offset adjustment and it is possible that you cannot null both offsets to be exactly zero at the same time. Go back to 10xDR.

Select R display, 2kW range and CAL mode and wait until the display has settled to 1000 and store this as the refer-ence voltage. Select the ZERO mode and switch to DR display. The difference should be 10x(0-1000) = -10000 digits. If not, trim with R206. Lift the SET REFERENCE lever. Negative readings are converted to zero refer-ence. Select the CAL mode. Display should go now to

10x(1000-0) = +10000 digits. If not, trim with R205.Select the R display mode and store reading 1000 as

the reference. The difference should now be 10x(1000-1000) = 0. The reading may differ by +/-5 digits even if the adjustment was made very carefully (we are dealing with analog circuits!)

The non-multiplied difference DR is easier to check and adjust. Set the DR/10xDR switch to DR, select R display, 200W range and CAL input. Store 10000 as reference. Change to ZERO mode and DR display. Use R202 if the difference is not -10000. Storing a negative value makes the reference equal to zero. Select CAL mode. Use R201 if the difference is not +10000 digits. Store this as refer-ence. The difference should be (10000-10000) = 0 digits, but an error of +/-5 is acceptable.

Restore the top cover. After an hour or so, check the self-calibration once again.

AVS-47B CALIBRATOR PLUG RESISTORS

*

J1

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

J2

CH0

CH1

CH2

CH3 k

CH4 k

CH5 k

CH6 M

CH7 -

CH0 has 200 leadres

*