HP3458A FLATNESS DAC CONVERGENCE 195 error

[Dr. Frank]

Wait a moment, I had a look on your picture.
You can see R303 and R304, right to U302, and both their soldering joints seem to be broken. Compare this to the solder joint of the blue capacitor to the left..
Or is it just a bad lighting in your picture?

Frank

[chilternview]

These 40mV between R302 and R303 should be exactly 1/10 of V_out, therefore this does not fit as well.

The junction of R302/3/4 was actually 0.040mV i.e. 40uV.

I measured the resistors with a 34401A:
R302 (1K) was 921ohms
R304 (10K) was 9.89Kohms
R303 (9K) was 2.7Kohms

Given that the output of U302 is shunted to earth by other resistors all of those look OK.

I also checked the readings with the DVM set to 10M and 10G input resistance, pin 2 read +95mV with 10Gohm input resistance, and +110mV with 10Mohm input resistance. It looks like R304 is OK.

About the solder joints, I think that's the light. I did try re-tinning the joints of the 3 resistors, no effect.

I can try replacing U302, not sure what else to try.

[Kleinstein]

One should be able to follow the signal path with a scope. So apply an AC signal to the 3458 and check the signal at U401 , U301 and U302. Unless the DAC is set very close to zero a copy of the input signal should be there.

Is the supply to U302 OK ?

[Dr. Frank]

Maybe you could check, if the supply voltages are ok.. or creating spikes..
Another U302 would mean, that the new one was damaged also, by overvoltage?
Does the old OpAmp still work properly on a breadboard, maybe at +/12V supply?

I'm baffled, meanwhile.

Frank

[Dr. Frank]

If the output is at -8.5mV, then 1/10 is about -850µV, but not +40µV, btw. That's not consistent, even on such low levels.

There's still something fishy in the resistor network, or the OpAmp draws too much current to its inputs.
What's the voltage across this 10k resistor?

Frank

[Kleinstein]

The LT318 can have quite some input bias current with typical 120 nA. So some 0.9 mV drop over the 10 K resistor is about normal - more on the low side.

Something like supply spikes are maybe a poor turn on sequencing possibly damaging the OP is a good point.

Parts of the amplifier are very fast and may oscillate at a frequency well above 100 MHz to fast to see with a slow scope. It is not very likely but can make parts behave strange.
Spikes on the -17 V supply could also cause some confusion.

[chilternview]

Thanks for the suggestions, so I tried applying a 10V 1KHz sinewave and setting the 3458A to DCACV. The value it measured was in the right ballpark (~3.5v).

A scope on pins 2 (inverting i/p), pin 3 (noninverting i/p) and pin6 (o/p) is attached. It looks like there is 10x gain from U302, with some spikes on the signal - these appear to be on the power lines.

Voltages measures were:

pin1 +11.71V      pin8 -16.5V
pin2 +114mV      pin7  +14.0VV
pin3 -0.4mV        pin6 -3.7mV
pin4 -17.75V       pin5 +11.73V

So the supplies are -17.75V and +14.0V. Are those OK?

[Dr. Frank]

If you also have bought a replacement for the 12bit DAC, then I would try it out.

Frank

[chilternview]

I am waiting for the DAC to try and see if that helps. In the meantime I tried measuring the voltage on pin2 of U302 for 4 different new LT318A's from Digikey, plus the original, HP-part number marked one.

The new ones measured from +120mV to +126mV. The original measured +65mV. Maybe the HP marked one was selected for low input bias current?

With R304 being 10K (I double checked by taking U302 out of its socket so R304 was floating under measurement)  then the input bias current can be calculated to be of the order of 12uA (or 6.5uA for the original part). Both are way over the datasheet spec of 150nA typical, 250nA max.

So it this high reading on pin 2 still makes no sense to me. And I'm not clear what the purpose of R304 is, either.

[Dr. Frank]

If the DAC has an output impedance of about 10k, then R304 would serve as an offset compensation for the bias current.

I still suggest to check the disassembled OpAmp on a breadboard if it works correctly.

Maybe you de-solder all 3 resistors, check them out of circuit and solder them again in place. I still do not trust these.

The bias current as well as the offsets cannot explained otherwise.

Frank

PS: the obsolete AD7240 has an output resistance between 7  to 15k, typ. 12k, so that's obviously the correct explanation about bias compensation.

I don't know, which replacement you have ordered, but the recommended AD7541 has a completely different pin out, requiring an adaptor.

[Kleinstein]

The DC readings at the U302 inputs is still very odd and can not be explained by the bias. Especially the +120 mV at pin 2 are strange.

A possible explaination would be that the meter used to measure the votlages effects the circuit: Additional capacitance (from the meter) at pin2 can cause oscillation of the OP amp. So when probing Pin2 one should have something like 100 K in series directly at the OP side to reduce the effect of the meter capacitance.

120 mV would also be enough to see with the scope. The scope trace from reply 32 shows much less than 120 mV.
The AC waveform shown from the scope suggest that U302 is working as intended at least for the AC part and also the DC part look OK with the scope.

[chilternview]

120 mV would also be enough to see with the scope. The scope trace from reply 32 shows much less than 120 mV.
The AC waveform shown from the scope suggest that U302 is working as intended at least for the AC part and also the DC part look OK with the scope.

Kleinstein, well spotted, why didn't I notice that!

With a 10X scope probe, DC coupled, there was no offset on pin2 (< 2mV) and no oscillation as in the pictures. When I connected the 34401A probe though, there was a 1V p/p ~500kHz oscillation! And of course the meter read a DC voltage in the tens of mV.

So U302 is OK and performing as expected. And I've learned a valuable lesson: always look with a scope if a reading is odd...

[chilternview]

One other question while I'm waiting for the replacement DAC:

Anyone know what the code at the end of the 'FLATNESS DAC CONVERGENCE 195' means?

I have seen others (e.g. TiN) report the error with number 198. Even someone with 197. But what does the code mean?

Has anyone disassembled the 3458 firmware so one could try and figure out what it's doing in that test?

[Dr. Frank]

that code is nowhere specified, unfortunately.
Even if there would be people who reverse engineered the code, they won't tell, to obvious reasons.

[Kleinstein]

The extra number probably is a slightly more specific number on which test failed. So 195 is likely an earlier failure in the flatness testing. This could be as little as a different range detecting the error, as the same DAC is used for different dividers.

I see 3 different types of error causing such an errors:
1) a problem with the DAC (it still looks like it works somewhat) or maybe the shift register
2) a problem with the test signal generation or maybe the measurement of the test signal (though this part should be tested in a different test, though that error may come later and may not show)
3) a problem with one of the dividers (drifting resistor or capacitor, bad JFET (or LM339) for switching), so that the DAC range is insufficient to get a good waveform.

If one has a good AC source or other meter one may be able to check the error in the AC ranges - chances are 1 or more of the AC ranges are out of spec, though not sure how much (could be just a few ppm or a few percent), either at low frequencies or higher frequencies (e.g. 10 kHz) or both. A gross error, like from a not working LM399 / JFET should be visible.

Another point to test could be to see how good the AC amplifier actually is, e.g. with a 1 kHz or similar rectangular signal, a bit like in the scope probe adjustment. Ideally this should give a relatively clean square wave also at the AC amplifer output. By tying the DAC output to ground one would get an intentionally bad setting.

[Traceless]

@chilternview:
Could you locate test point 901 on the A1 board, monitor it with your oscilloscope during ACAL AC and post a screenshot here?

[chilternview]

@chilternview:
Could you locate test point 901 on the A1 board, monitor it with your oscilloscope during ACAL AC and post a screenshot here?

I'm not sure if it helps, as there is a *lot* going on during an ACAL AC. Here is a bit caught just after the ratio tests.

[chilternview]

If one has a good AC source or other meter one may be able to check the error in the AC ranges - chances are 1 or more of the AC ranges are out of spec, though not sure how much (could be just a few ppm or a few percent), either at low frequencies or higher frequencies (e.g. 10 kHz) or both. A gross error, like from a not working LM399 / JFET should be visible.

I ran the output of a HP3245A set to 10VP/P AC sinewave via coax cable to the 3458A (HP3458A-2 below) and another, Keysight-calibrated 3458A (HP3458A-1 below). Here are some measurements. The range was set manually on the ACV reading.


Freq 1KHz           HP3458A-1        HP3458A-2
---------------------------------------------------
Range: 1KV         0003.687          0003.811
Range 100V         003.5366          003.5779
Range: 10V         03.53392          03.53555
Range: 1V           OVLD                3.421325

Freq 10KHz
----------------------------------------------------
Range: 1KV         0003.690          0003.781
Range: 100V        003.5411          003.5827
Range: 10V         03.53978          03.55297
Range: 1V           OVLD                3.575434

Freq 100KHz
----------------------------------------------------
Range: 1KV         0002.461          0003.157
Range: 100V        003.4822          003.5705
Range: 10V         03.53122          03.57405
Range: 1V           OVLD                3.593496

Freq 1MHz
----------------------------------------------------
Range: 1KV         0001.421          0001.361
Range: 100V       001.7199          002.2459
Range: 10V         03.0461           03.11238
Range: 1V           3.148369          3.169615

[Traceless]

@chilternview:
Could you locate test point 901 on the A1 board, monitor it with your oscilloscope during ACAL AC and post a screenshot here?

I'm not sure if it helps, as there is a *lot* going on during an ACAL AC. Here is a bit caught just after the ratio tests.

Hello chilternview,

thanks for posting the scope screenshot. What we are looking for is a square wave output with peaks > 10V and < -10V (which is outside the ADC operating range) immediately before the meter outputs the "error beep sound". If that happens *thoroughly* check Q802 (don't rely on one of the cheap IC testers). That should be an 2N4392-N-channel JFET.

That being said while the signal on your screenshot is still within the +/-10V range the part I marked red on your screenshot does look suspicious to me. I'm not sure what is happening at that point in time but, the -5.5 - -6V peaks look odd. It would probably be helpful if someone with a working 3458a could provide an ACAL log of TP901 for reference.

Cheers Traceless

[chilternview]

Thanks traceless,

The bit captured in the following was by single shot triggering on +9V rising edge. At approx t=6s after triggering, the beep indicating the error occurs.

There are +12 / -15v peaks then +3/-3v levels (approx 31 of each) - with spikes of varying amplitude. Not sure if this is revealing anything useful.

I have seen someone else refer to an issue with Q801/2 elsewhere, can't think where. I do have a couple of spares so it may be worth checking them as you say - I believe that part of the circuitry is generating the cal voltage input?

[chilternview]

Tried replacing both Q801 and Q802, alas no difference...

[Kleinstein]

The TP901 signal is the signal going to the ADC in fast mode. For the compensation test this would likely be the output of the S&H stage.
The rather large overshoot does not look very good, but is quite short and may be OK. It is not so clear at what time the actual ADC conversion is done.
For comparison one could looke at the AC main amplifier output (TP401) to see if the large overshoot is an effect of the AC amplifier or the S&H path. For the main amplifier that much overshoot would be suspicious. For the S&H part it may be OK, if only short.

What is a bit strage is that the amplitude for the +3-4 V signal varries quite a bit between the pulses, while the negative side look more or less stable. This suggest that there is quite some effect of the compensation settings for the positive side only. The neg. side may be fixed just to test the other sign too and no real search for the best solution there.

The AC test shows surprisingly large errors even for the "good" meter. Chances are the 3.5 V eff are borderline for the 1 V range, so the overload result is not a significant result. To test the smaller ranges it would need a smaller amplitude (e.g. some 100% of nominal range). The question would be if the low frequency (e.g. 100 Hz) and higher frequency (e.g. 10-50 kHz) readings are similar in ratio. The DAC for the flatness should effect the low frequencies below some 2 or 8 kHz.
The test in the 1000 V range and a bit already the 100 V range are on the low side for the amplitude, especially for the analog RMS case. So the difference there could be an effect of the RMS chip.

[chilternview]

Sorry Kleinstein, that *was* TP401 that the scope was connected to i.e. the amp output.

[Kleinstein]

For the amplifier output the levels at some +12 and -15 V are about the limits on what the amplifier can output in saturation. The test signal is somehow generated idurectly via flip flops and that may cause the extra phases at the start. So no problem with the asymmetry there and the pulses itself may also be no problem.

When reaching the slew rate limit the amplifier may be a little asymmetric can slower in one direction than the other. So asymmery in the overshoot is not a big surprize.
A possible reason for higher than normal ringing could be an error in the amplifier compensation adjustment, the part that goes via CR402 and seems to be not part of the normal ACAL. So a wrong value in the CAL RAM could cause this. A rather small voltage at the cathode of CR402 / output of U402B would be a bit suspicous.

The ringing may as well come from the test signal and may not be an issue at all.  One could check this with a a good square wave at the input.

[chilternview]

For the amplifier output the levels at some +12 and -15 V are about the limits on what the amplifier can output in saturation. The test signal is somehow generated idurectly via flip flops and that may cause the extra phases at the start. So no problem with the asymmetry there and the pulses itself may also be no problem.

When reaching the slew rate limit the amplifier may be a little asymmetric can slower in one direction than the other. So asymmery in the overshoot is not a big surprize.
A possible reason for higher than normal ringing could be an error in the amplifier compensation adjustment, the part that goes via CR402 and seems to be not part of the normal ACAL. So a wrong value in the CAL RAM could cause this. A rather small voltage at the cathode of CR402 / output of U402B would be a bit suspicous.

Thanks, I'll take a look.

The ringing may as well come from the test signal and may not be an issue at all.  One could check this with a a good square wave at the input.

I have tried running a square wave of amplitude 0.1/1/10v and frequencies of 100Hz-1MHz and the waveform at TP401 looks pretty good - some sloping at 100Hz from LF rollof, very good from 1KHz to 100KHz and some HF degradation above 100KHz.

I don't see much signal amplitude on the REF input of the DAC U301, what sort of voltage (DC or AC?) should be expected here?