We are using the ACS711 Hall effect sensor and interfacing the VIOUT pin to an ADC. I need to know the source impedance (internal) between the internal opamp that produces this signal and the device pin. I cannot find this in the data sheet, and Allegro referred me to our supplier which is Digikey. Is this information available?
Hi ed.jaeger
I will need to check with the product specialist for this information. Do you have the complete manufacturer part number you are using?
Thank you, yes. It’s the ACS711KEXLT-31AB-T
Hello ed.jaeger,
In an ideal world, the output impedance of an op amp is zero.
As I understand your questions, you are asking about the resistance between the op amp output as installed on the semiconductor die and the pin as it connects to the PCB. This would include the resistance of the internal bond wires or other such internal connections.
Agree, this information is not in the data sheet. IMO, this is a highly specialized question that would require knowledge of the internal chip design.
This appears to be a black box (hidden) problem. Recommend an imperial Thévenin measurment:
- configure the circuit for normal operation
- apply a known current to the input
- measure the unloaded output
- measure the output when the op amp output is loaded by 10 kΩ and then 1 kΩ resistors
- compute the Thévenin resistance
Sorry, I couldn’t assist more.
Best Wishes,
APDahlen
P.S. Assuming you are using a sample-and-hold ADC, does your design have enough timing flexibility to accommodate a non-zero impedance?
Hello APDahlen, thank you for the reply.
Yep, clearly it should be 0Ω but I’m sure it’s not It could be low, like 50Ω or so, it could be higher like 1kΩ, it could be something else. The data sheet does indicate a minimum load resistance of 15kΩ.
The method you describe is exactly what I was planning to do if I could not get the actual figure, so that’s what I’ll do - and I’ll post my results as soon as I can figure out how to do it with that little part. I see there’s a demo board so I’ll probably get one of those.
It is a sample and hold and I have all the information on the internal connections - interconnect resistance, input and SH capacitor values, multiplex switch, etc. I’m sure I’m just being overly cautious, but since there is a calculation for sample time that requires as input the source impedance, I’m looking for it.
Appreciate your assistance, it’s a really cool part.
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Absolutely, with such a high bandwidth!
Let me thank you in advance, I truly appreciate you posting the results.
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Not easy to measure on our board, so I’ve ordered a couple of the breakout boards to use on the bench. Will be sure to check back.
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Thank you for the update.
Looking forward to seeing your results.
r/
APD