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Replacement for Fluke 700013 IC (quad SPST analog switch)

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JoergR:
Hm, the ADC itself is most certainly not broken. When putting a short across TP103 (the output of Track and Hold) and Reference Low, I get close to zero counts, 16 to be exact in the 2V range (everthing below 35 would be fine). When putting a voltage below 2V between TP103 and Reference Low in the 2V range, the display shows the correct value with opposite sign, just as should be according to the manual. Changing ranges just changes the decimal point in the display, but not the digits.

I do think U301B is a problem due to the 2V range not working and I guess that's what brought me here in the first place. But let's leave that aside for the moment and focus on the Track and Hold if we can.

The input to the Track and Hold does appear to be working as intended in the 20V range. See the attached images. It oscillates between 1/10 of the input signal and GND as should be and no factor of 2 to be seen. This is true for both positive and negative input voltages.

One can also see the output of the track and hold in comparison - the high part of the signal is not amplified, but the low part of the signal is for positive voltages and the opposite is true for negative voltages.

The Z303 measures correctly. The digital inputs on U303 I measured and they correspond to my functional device. I wonder if some of the capacitances in T&H could be at fault or the current limiter (?) CL301 - it's high side is at zero volts. I'll make more comparisons with my functional device.

Kleinstein:
The ouput of CL301 at around 0 V is about OK. It should about follow the input to the S&H stage with a little offset in the  0 - 1 V range.
If needed one could check the current from CL301 by looking at the drain voltage / input to the OP to see how much current is flowing.


As the track and settle phase seem to work I would expect the problem in the hold phase.   
It somehow looks like the signal in the hold phase is about doubled for some reason - which is odd, as there are no 2 equal resistors to make it double.
I see absolutely no reason how a broken capacitor could double the voltage - leakage would more like reduce the voltage. It would be more like having the switch U303 B not turning off. The divider would than be with 2 of the switches. 
The low side of C308 may tell the difference.

JoergR:
Indeed, the signal on the low side of C308 - measured at U303-13/14/16 is different from the functional device. See the scope shots. It also shows a larger amount of ripple. But at this point, I'm pretty convinced that the switch ICs need replacements. I'll desolder them and see if I can pin down the defective switches and thus make sense out of the behavior observed. Thanks a lot to kleinstein for being so helpful!

JoergR:
So here is the first one - U303. Only switch A seems to work as needed. Switch B never opens when a static high is on the latch and B, C and D drop about 0.6V when closed - indicating an on resistance of 1.7kOhm which seems a little excessive, but it could explain the false readings.

Kleinstein:
1.7 KOhms sounds rather high for the on resistance.  With the large area used for the swtiches I would habe expected lower resistance, though the use in the ohms past suggests that the switches are pretty low leakage.  AFAIK the switches have a NMOS and CMOS part in parallel. The 1.7 K could be just one of those 2 halves and would than depend on the common mode voltage.  There is a slight chance the other half of the switch was damaged by overload from driving it too hard. Another possibly explaination would be some supply missing (e.g. broken bond wire or trace) so that maybe one polatiry never truns on and the other never truns off.
 
At least the 1.7 K would be high enough to explain why it still kind of worked for a gain of 2. The other involved switch would likey be similar resistance.


For the replacement it may be interesting to know of the latches are transparent latches reacting on the level like the 74xx75 (though other polarity) or D-type flipflops (like 74HC74) reacting on the transitions only. The chips are connected relatively direct to the CPU bus and thus get valid data only for a relatively short time.
The 74xx75 would definitely need an inverter for the latch signal, but still not so sure it would be right or if something like 74AC175 may be better.

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