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Op-amp input impedance when unpowered
Op-amp input impedance when unpowered
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Old 6th June 2021, 07:12 PM   #1
newvirus2008 is offline newvirus2008  India
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Default Op-amp input impedance when unpowered

Hello all,

I was wondering what the input impedance of an op-amp like LM4562 would be, in the absence of a power supply (0V). My wild guess is that it would still be high (not sure though), as the input stage biasing would be lost, deactivating the differential pair, muting the amplifier. Please correct me if you think I'm wrong.

The basis for the question is the idea of a simple attenuation ON/OFF switch that would short out the power supply of an inverting attenuator (using MOSFET/relay etc.), to turn off the attenuation (picture). After removal of power, the signal path to the next stage (power amp) is expected to be retained through the resistors Ri and Rf.

Please note that I would like to avoid using controls / switches or digital attenuation for varying the gain, as the number of channels is just too many (16). Looking forward to reading your comments. Thank you.

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Old 6th June 2021, 08:36 PM   #2
analog_sa is offline analog_sa  Europe
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Op-amp input impedance when unpowered
Input impedance will be unpredictable and non linear, likely leading to increased distortion in the off state.

The idea works well with triodes when the heater voltage is used as an on/off control
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Old 6th June 2021, 09:51 PM   #3
tomchr is offline tomchr  Canada
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+1 Nonlinear and non-predictable.

The only thing I would predict is that you likely get a diode clamp at the input due to the ESD structures within the chip. That'll limit the input voltage at the pin so somewhere around 0.5-0.7V.

If you want to make a switchable attenuator, why not just have a relay that bypasses the attenuator? Or controls the gain if you're looking for switched gain settings.

Tom
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Old 6th June 2021, 10:46 PM   #4
Potentiallyincorrect is online now Potentiallyincorrect  United States
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Almost all IC's have input protection diodes to V+ and V-. As such things get very nonlinear. IC's can actually become powered up by by the input signal. This can become very confusing especially with digital IC's and be difficult to debug.Other conduction paths depending on the input pair type and topology may also become involved. Don't expect the input source to to not see peculiar loading. At least with the inverting topology you have chosen the Zin will not fall below Ri.
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Old 6th June 2021, 11:54 PM   #5
nigelwright7557 is offline nigelwright7557  United Kingdom
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Not sure how wise it is to put volts on an op amp input with no power.
It could latch up and cause problems.
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Old 7th June 2021, 05:56 AM   #6
newvirus2008 is offline newvirus2008  India
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Thank you everybody for the responses. I am not very good with the insides of ICs, but I guess the diodes you mention are between + & - inputs (differential clamp), with ESD diodes from each pin to V+ & V-, correct? Now I can see what I was missing.

Quote:
.. why not just have a relay that bypasses the attenuator?
Going that way, all channels would have to controlled together, with as many switches.

Since the power supply idea is almost guaranteed to fail, I could try CMOS quad switches like 74HC4066. But are there any tricks to linearise them so that they could be used as part of Ri or Rf, to switch gains as mentioned above? Or should I use them to bypass the attenuation circuit somehow?

Last edited by newvirus2008; 7th June 2021 at 06:06 AM.
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Old 7th June 2021, 07:45 AM   #7
newvirus2008 is offline newvirus2008  India
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Based on your comments above, I came up with the switched gain circuit in the picture below. For a 10V (+/-5V) supply, the non-linear error per switch is supposed to be ~5 ohms, while the off-state leakage is below -100dB at 20kHz, as per the 74HC4066 datasheet.

IMG_20210607_132215.JPG

Although this one could work, I'm still not sure about its performance. Do you people see any issues relating to noise, distortion etc. with this circuit? Thanks.

Last edited by newvirus2008; 7th June 2021 at 07:47 AM.
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Old 7th June 2021, 01:34 PM   #8
jan.didden is offline jan.didden  Europe
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The opamp inverting input is a virtual earth. So when you move the switches to the opamp input, they will have no CM voltage when closed which lowers their distortion dramatically.

Jan
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Old 7th June 2021, 04:49 PM   #9
newvirus2008 is offline newvirus2008  India
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Yes Sir, I think I understood what you said (picture). However, I am not sure I would go with it, as have a very bad feeling about using amplifiers with Rf < Ri. Thank you very much for the tip.

IMG_20210607_222809.JPG
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Old 7th June 2021, 05:02 PM   #10
jan.didden is offline jan.didden  Europe
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That's not what I meant, sorry to be unclear, I meant putting the 1k on the Vi side (post # 7) so the switch is at the virtual earth.

BTW Rf<Ri just means that the gain<1.

Jan

Last edited by jan.didden; 7th June 2021 at 05:04 PM.
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