Products > Test Equipment
Replacement for Fluke 700013 IC (quad SPST analog switch)
richipedia:
--- Quote from: JoergR on June 03, 2021, 07:21:46 pm ---Glad you join the discussion again richipedia.
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I guess that's what people do with a 3 year old thread about an unfinished project ;)
--- Quote ---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.
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So why choose the normally closed version of the chip for the replacement?
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Please take a moment to look at my schematic: https://www.eevblog.com/forum/testgear/replacement-for-fluke-700013-ic-(quad-spst-analog-switch)/?action=dlattach;attach=1139450
The control signal for the analog switches comes from a NAND gate, so the switch should be conducting when the control signal is low and vice versa.
I should check what exactly happens at power on, to make sure that no opposing switches are turned on at the same time.
--- Quote ---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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Maybe the latched inputs on the devices you checked do not change in between cycles. The fact that the switch output changes before the clock edge might be a hint that only the unlatched inputs have changed.
Kleinstein:
Chances are high that both latiches and D-Fliflops would work. The switches are accessed similar to memory. So there is a CS pulse to activate the chip if a suitable address (actually a large block) and the control signals are there. The data should than be valid for a relatively short time (likely 0.2-1 µs range) and in this time the latching signal should get a short pulse. The difference between a latch and Flipflop would be when the switched would change: with the latch the change would be just after the start of the pulse and the signal at the end of the pulse (which should be the same as at the start) would be preserved.
With a Flipflop the data just befor the end would be transfered to the switch - so a slightly later change, but essentially the same result. The exact timing (10s of ns range) may be a bit different. It could make a difference with timing just at the edge -- however than it is not clear how fast the fluke chip is.
Both DG211 or DG212 could work - just need to have the suiteable logic chip in front (74xx00 = NAND or 74xx08 = AND).
The start up condition may indeed be a bit tricky. It may take quite some time until the CPU initiallizes the switches. So the initial state may be there for quite some time and multiple switches on could cause trouble.
JoergR:
I had seen that you drive the switches with a NAND and so your design is fully equivalent in function - it couldn't function otherwise. I just wondered why you choose the combination NAND + Normally Closed switch over AND + Normally Open switch - just availability of chips or was there a deeper reason behind that? In fact, one could solder AND + Normally Open switch into your PCBs and it would work just the same, wouldn't it?
The difference may come only when someone applies a voltage on the input of the DMM without the meter being powered - then it might make a difference? That could occur if someone turns the multimeter off while the inputs are still connected or has forgotten to turn it on when the test circuit is already connected etc.
richipedia:
--- Quote from: JoergR on June 03, 2021, 08:26:58 pm ---I just wondered why you choose the combination NAND + Normally Closed switch over AND + Normally Open switch - just availability of chips or was there a deeper reason behind that?
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An AND gate is made up of a NAND gate followed by an inverter, so the NAND gate has shorter propagation delay.
One could argue that a DG211 might be a DG212 with extra inverters before the inputs or vice versa, I honestly don't know.
Using the NANDs and the DGx11 seemed more natural to me, 3 years ago, but anyhow, we're already splitting hairs. :)
--- Quote ---In fact, one could solder AND + Normally Open switch into your PCBs and it would work just the same, wouldn't it?
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Absolutely.
--- Quote ---The difference may come only when someone applies a voltage on the input of the DMM without the meter being powered - then it might make a difference? That could occur if someone turns the multimeter off while the inputs are still connected or has forgotten to turn it on when the test circuit is already connected etc.
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I believe that the difference between the DG211 and the DG212 is basically that one of them has an extra inverter at the digital control input whereas the other doesn't. The actual switches are probably made of ordinary enhancement type FETs - no use of fancy depletion type FETs for the "normally closed" type, at least in my opinion. The terms "normally open" and "normally closed" probably are misleading.
Also, I just measured one of my DG411 ICs: switch resistance is ca 20 MOhm when unpowered :)
edavid:
--- Quote from: richipedia on June 03, 2021, 08:45:15 pm ---I believe that the difference between the DG211 and the DG212 is basically that one of them has an extra inverter at the digital control input whereas the other doesn't.
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Usually the switch cell is driven by complementary signals from the input buffer. To change the control signal polarity, those drive signals are routed to the opposite inputs of the switch cell. No extra inverter is needed, just a wiring change.
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