Protect SD ADC input from ina output voltage swing? - Page 1
Forum section: electronics / beginners

author: julian1
author_rank: Frequent Contributor
timestamp: February 10, 2021, 07:13:38 pm
content: Imagine this scheme.pluggable thermocouple -> connection block -> ina -> sd adc.ina supply +-12Vsd adc supply +-2.5V. (input voltage limit AVDD+ 0.3V/ AVSS-0.3V).The thermocouple is user detachable. If it is not attached (open circuit), the ina +input goes high-Z, and the ina output may swing to the +-12V rails.This exceeds the adc input voltage spec, and ina output current will sink via the internal adc ESD protection diodes.How to mitigate,- Use schottky/PN diodes between ina and adc to dump current to supply rails before the sd/adc ESD diodes. (issue - undesirable in the signal path/ schottky leakage etc).- run the ina on +-2.5V+output headroom, to limit output voltage swing to adc input range. (issue - ina output swing/headroom is poorly specified )- put an analog switch (100ohm on impedance) in front the adc (130k input impedance at Fmod). Adc has internal comparators/registers to monitor OV condition,So use mcu to switch off adc input when adc detects overvoltage condition. So adc ESD only has to deal with a +-12V transient eg. 1ms. (issue is that protection relies on software, not hardware, but otherwise seems simple).I like the third approach. But maybe there is a another/better way?Edit.After posting, it occurred to me that it's not very nice if the ina goes into saturation, regardless of whether the adc is protected. I was intending to use an input relay to handle this. But if the relay was dpdt, then it could alternatively switch between the thermocouple inputs and agnd, and thus keep the ina output/adc input within range. So the mcu monitors the adc over-voltage register, and then switches the input from thermocouple to agnd - to avoid the high-Z condition, if the thermocouple is not connected. Transients are limited to the relay switching time.

author: mvs
author_rank: Frequent Contributor
timestamp: February 10, 2021, 09:40:30 pm
content: You can use a low voltage zero-drift opamp (like max4238) instead of an instrumental amplifier. You will need then precision resistors for feedback, perhaps even matched network.

author: thm_w
author_rank: Super Contributor
timestamp: February 10, 2021, 10:20:50 pm
content: I'm not sure how the analog switch option would be fast enough, unless you have some large RC filtering leading up to the ADC. Although you may want that anyway.Traditional approach is this one:" - Use schottky/PN diodes between ina and adc to dump current to supply rails before the sd/adc ESD diodes. (issue - undesirable in the signal path/ schottky leakage etc). "Using R/C and something like BAV199. I'm sure you can find lots of example circuits online:https://www.ti.com/lit/ug/tidu574/tidu574.pdfIf you use large enough R then you can just rely on the internal ESD diodes (make sure to clamp the supply voltage elsewhere though).

author: julian1
author_rank: Frequent Contributor
timestamp: February 10, 2021, 10:36:24 pm
content: Quote from: thm_w on February 10, 2021, 10:20:50 pmThis is what I was hoping to get away with (in combination with monitoring adc OV comparator and relay switch-off).+-2.5V supply for the adc was initially just an op-amp. Then changed to opamps with bjt followers in anticipation of need to sink higher current - from adc internal ESD diodes.

author: KT88
author_rank: Frequent Contributor
timestamp: February 10, 2021, 11:02:45 pm
content: You could use a low voltage inamp like the AD8237https://www.analog.com/en/products/ad8237.html.This inamp doesn't suffer from the diamond shape limitations and comes with RFI filters. It is recommended for TCs (P. 24 DS).In- or opamps can usually take 5-10 mA permanently and up to 100mA for a ms if not repetitive.You only need some low leakage diodes and siutable resistors to limit the currrent into the inamp. At 2.5V supply your ADC would be safe...CheersAndreas

author: David Hess
author_rank: Super Contributor
timestamp: February 12, 2021, 12:49:53 pm
content: Quote from: julian1 on February 10, 2021, 07:13:38 pmThe output resistance of the instrumentation amplifier is low and well defined so why does schottky diode leakage matter?A pair of diodes, or more likely a transistor and diode or two transistors, can be used in place of a diode clamp with one compensating for the Vbe of the other to make a clamp with low offset. This also has the advantage of being lower leakage than a schottky diode or very low leakage with the right transistor or diode.QuoteWhy does saturation matter? It will not damage the instrumentation amplifier and it does not seem like recovery time matters in your application. Sometimes saturation is undesirable because of higher current draw.An alternative which will both clamp the output and prevent saturation has the output clamp provide feedback to the input to limit the output range however offhand I do not know of any instrumentation amplifiers which can easily be configured to take advantage of this.

author: julian1
author_rank: Frequent Contributor
timestamp: February 12, 2021, 07:18:19 pm
content: Quote from: KT88 on February 10, 2021, 11:02:45 pmThe 5V supply means it drops into place, on the +-2.5V supply for the adc, and the r2r output matches the input range of the adc. In my mind I think of 5V and r2r ops as being kind of exotic, but maybe I need to change my approach.

author: julian1
author_rank: Frequent Contributor
timestamp: February 12, 2021, 07:36:00 pm
content: Quote from: David Hess on February 12, 2021, 12:49:53 pmYou're right. ina is low impedance output so leakage does not matter. Not sure why I got hung up on this. I will change it.QuoteIt seems like poor engineering. But pragmatically maybe it is just not an issue (And it is only temporary, until the input can be switched off). Current draw, or recovery time is not important.I understand how to clamp an op-amp output with negative feedback and a couple of diodes/zeners. But not sure if the same approach is feasible with a discrete ina, where the -ve input off the (front internal) diff-amp is not pulled out to a package pin.

author: David Hess
author_rank: Super Contributor
timestamp: February 12, 2021, 09:03:06 pm
content: Quote from: julian1 on February 12, 2021, 07:36:00 pmAll modern parts should be well behaved when their output is driven into saturation. Some old parts will draw excessive but not damaging current which might be undesirable in low power applications. Some old dual and quad parts would misbehave if one amplifier was driven into saturation.QuoteThe point of a discrete instrumentation amplifier is that the internal inverting and non-inverting inputs are available so clamping to the inverting input is available just like any operational amplifier. Note that real instrumentation amplifiers have high impedance inputs and typically do *not* use the common three operational amplifier configuration, but they still include an operational amplifier as an output element. A discrete implementation of these is made using a matched dual or quad transistor and operational amplifier. I now try to distinguish "true" instrumentation amplifiers by referring to the others as "difference" amplifiers.Some operational amplifiers have external clamp inputs or provide access to a compensation pin which can be used to clamp the output like the LM301A does. An instrumentation amplifier based on the LM301A can use external compensation pin 8 to clamp its output.Another way to implement clamping is to limit the output current of the operational amplifier into a resistive load; this can be done with a diode bridge and a current source and current sink. Or I have seen some designs which used an amplifier that supported shutting down or disconnecting its output.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 13, 2021, 11:25:15 pm
content: Imagine this.. you posted a circuit diagram with some labeled parts.If overloaded, the ADC is going to suffer longer than expected recovery times because of its sampling and filtering schemes.One straightforward scheme is to drive the ADC from an op-amp running on the ADC supply rails- then protect the op-amp input from over voltage- it will recover much faster than the ADC. Consider using a dedicated ADC driver amplifier that is designed to feed the weird sampling input structure.Better still use a thermocouple ADC with burnout detection eg ADS1120. Or if you like your analog and have cash to burn use an AD594!

author: julian1
author_rank: Frequent Contributor
timestamp: February 14, 2021, 11:33:59 am
content: Quote from: Terry Bites on February 13, 2021, 11:25:15 pmThat burnout current source mechanism is clever. First, turn on the 10uA source. Then open the relay to the thermocouple. If the thermocouple is detached and open circuit, the 10uA keeps the ina output in check, and we know it's open-circuit from the ADC reading at max_value (or reading the adc OV comparator).Otherwise if the thermocouple is attached (given from ADC reading), then turn off the burnout current source, so as not to influence the actual measurement.That may be feasible to implement, and may even avoid the need for a front-end switching relay/analog switch entirely. ie. Always keep burnout source active, unless adc reading indicates the thermocouple is definitely attached. The only issue I see is - what switch off leakage current is acceptable.QuoteI've actually got an integrated analog cold junction compensation part around here somewhere. But I suspect it's a more complicated approach than just digitizing (a local thermistor, on a separate adc channel) and doing the calculation in software.