Author Topic: How to measure voltages in high Z circuits ?  (Read 412 times)

Vivek

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How to measure voltages in high Z circuits ?
« on: July 31, 2022, 02:09:35 PM »
On a physical Rockman Sustainor, how does one measure the voltage on capacitor C1 as a function of time ?

I have a scope with 1M inputs. It will load the circuit since the input impedance is way smaller than R3 = 11.1 Megs.


PRR

Re: How to measure voltages in high Z circuits ?
« Reply #1 on: July 31, 2022, 05:49:30 PM »
TLO71 buffer.
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Rob Strand

Re: How to measure voltages in high Z circuits ?
« Reply #2 on: July 31, 2022, 07:17:28 PM »
TLO71 buffer.
You will need a dual supply (or equivalent) if you want to measure down to zero volts, and in your case negative voltages.
Otherwise the output won't be correct due to output swing and input common-mode range limits.
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PRR

Re: How to measure voltages in high Z circuits ?
« Reply #3 on: July 31, 2022, 09:05:46 PM »
You will need ...

I think Vivek is experienced enough to figure that out; don't you?

It is a very incomplete problem description.
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Rob Strand

Re: How to measure voltages in high Z circuits ?
« Reply #4 on: July 31, 2022, 09:14:38 PM »
Quote
I think Vivek is experienced enough to figure that out; don't you?

It is a very incomplete problem description
I don't know.

It's easy for pedal builders to get caught-up on single supply configurations since they deal with AC.

Same goes for bias currents.  It might seem OK to use any old opamp for a buffer but the reason the TL07x is the choice is because of the JFET inputs.   A bipolar input opamp would cause a DC offset through the 11M.

It's all obvious if you have had to deal with precision DC measurements.
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