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Replacement for Fluke 700013 IC (quad SPST analog switch)
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richipedia:

--- Quote from: Kleinstein on June 01, 2021, 06:34:15 am ---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.
--- End quote ---
Thank you for pointing that one out. In my opening post, I already mentioned that the original 700013 are transparent while the latch input pin is low. However, for my replacement, I chose 74AC175, which are flip flops, not latches.  |O
On my first veraboard prototype, I used 74AC573 latches, which do the job.

However, I do not know if the difference in behavior between latch and flip-flop is relevant.
richipedia:

--- Quote from: Kleinstein on December 27, 2020, 06:27:02 pm ---The DG411 is low resistance, but also has more leakage. The DG211 should be a lower leakage counterpart - AFAIK with the same pin-out  :-*. Leakage can be a problem with the higher resistors.

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I just tried to get an estimate for leakage current using a power supply at 5 V and a DMM in mV mode. The DMM has an input resistance of R_M=10 MOhm, so the voltage reading V_M should give me an indication of the leakage current I_L = V_M / R_M.
The DG411 switch I measured has ca. 0,05 nA ... 0,11 nA of leakage, which matches the typical figure in the datasheet.
On the other hand, the 700013 I measured has ca. 6 pA ... 15 pA of leakage. One order of magnitude less, just like the DG211. That's a good excuse to order some DG211.

Thank you again, Kleinstein!


Does anybody have a clue how relevant switch charge injection is for this application?
Kleinstein:
I don't think the charge injection should matter very much. It may make a small difference at the S&H stage, but I would not expect very much difference as the capacitor is quite large (470 nF) - less charge injection should still be better. Chances are there is a correction parameter to compensate from the calibration.

The 74AC573 should be essentially the 6 channel equivalent to the 74HC77/75. It should be active high and thus the wrong polarity.  A tranparent latch has a kind of hold and an a transparent mode. The D-flip flop takes over the input only at the transition, and thus no tranparent phase, but a defiend time to copy the input data.
The HC175 should read the data at the positive edge and thus at the end of the  /CS2 or /CS3 pulse.  As far as I understand the bus protocoll this should be OK. However I don't know the bus with the /DS  /DM signals.

The /CS2 / CS3 signals come from an 74LS... , at least in the plan.  So the latch / flipflop should be a HCT or ACT version. The AC / HC version may no reliably detect the high level (an extra pull up may help as a quick fix).
richipedia:

--- Quote from: Kleinstein on June 03, 2021, 03:39:58 pm ---The 74AC573 should be essentially the 6 channel equivalent to the 74HC77/75. It should be active high and thus the wrong polarity.
--- End quote ---
I corrected for the wrong polarity using an inverter :)


--- Quote ---The /CS2 / CS3 signals come from an 74LS... , at least in the plan.  So the latch / flipflop should be a HCT or ACT version. The AC / HC version may no reliably detect the high level (an extra pull up may help as a quick fix).

--- End quote ---
Another big trap for young players. The Z8 microcontroller also uses TTL voltage levels.

I think it's time for me to make a new replacement, using HCT, ACT or even LS logic and the venerable DG211 :)
I might also make the board a little smaller. If i had the AC option PCB, it would collide with the replacement boards.
JoergR:
Glad you join the discussion again richipedia. I'm learning a lot here about logic families - much like Montagues and Capulets it seems...
One question I still have:

--- Quote from: softfoot on May 29, 2020, 08:57:06 pm ---BTW is the 700013 normally open (ie if an input is LOW is the switch OPEN) ??? 
If that is the case then the DG212B (normally open) would be best because the DH211B is normally closed.

--- End quote ---
So why choose the normally closed version of the chip for the replacement?

For what its worth, when I checked the input signals to the 700013 chips in my devices, they would always switch several microseconds before the clock signal goes low and would stay at their level several microsecods until after the clock goes high again. From that, it seems that D-Flip-Flops would be just fine. But then, I only thoroughly checked in 20V mode and it may be different in other ranges.
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