Op amp maximum input voltage

Thread Starter

ScotsDon

Joined Jun 15, 2020
2
Hi all. I have just joined. Many Precision Rectifier Single Supply Op Amp circuits feed the input AC waveform into the non-inverting (+) terminal of an op amp (e.g. see https://www.rlocman.ru/i/Image/2015/10/01/Fig_1.gif). Say Vcc = +15V, Vee = 0V, and Vin is an AC input of +/- 5V (10V peak-to-peak), then when Vin is in the negative half-cycle, the first op amp outputs zero volts (it can't go lower), so input V(-) is also 0V via the feedback. Input V(+) however is controlled by the input AC voltage and will fall to -2.5V at its minimum. Now most op amp specs say under Absolute Maximum Ratings: Input Voltage "Vee-0.3V to +32V" say). My question is, surely Vin at -2.5V is outside of the Absolute Maximum Rating of -0.3V? Also the Common Mode input at -1.25V is outside. So am I missing something? Any light anyone can shed on this will help a lot.

See
https://www.radiolocman.com/shem/schematics.html?di=161475
for the complete article.

ScotsDon
 

Papabravo

Joined Feb 24, 2006
14,701
With single supply circuits you normally bias one input at Vcc/2 if Vee is a GND. The AC signal is coupled through a coupling capacitor that has a "low" impedance at the frequency of interest. You may or may not need a coupling capacitor on the opamp output as well. Again it should be low impedance at the frequency of interest.
 

Thread Starter

ScotsDon

Joined Jun 15, 2020
2
Very true. However, the example circuit does not shows coupling capacitors and I was wondering how it (and similar ones) worked, without endangering the op amp. FYI, I have made a precision rectifier work in the way you suggest, except it leaves my rectified DC signal floating at the mid-point! I probably need to add a subtractor to bring it down to earth. Thanks for reply. Any further info is welcome.
 

crutschow

Joined Mar 14, 2008
25,683
I was wondering how it (and similar ones) worked, without endangering the op amp.
The Opamp substrate diode conducts in the reverse direction.
That typically doesn't hurt the op amp if the current is limited to a few mA.
I would use a higher value of input resistor, like 10k to minimize the current.

Below is the LTspice simulation of two other single-supply, precision rectifier circuits beside the one you referenced.
Note that the two with diodes prevent the input from going negative due to the feedback.

Bonus points if you figure out how they all operate. :cool:

1592254260727.png
 
Last edited:

Analog Ground

Joined Apr 24, 2019
420
Hi all. I have just joined. Many Precision Rectifier Single Supply Op Amp circuits feed the input AC waveform into the non-inverting (+) terminal of an op amp (e.g. see https://www.rlocman.ru/i/Image/2015/10/01/Fig_1.gif). Say Vcc = +15V, Vee = 0V, and Vin is an AC input of +/- 5V (10V peak-to-peak), then when Vin is in the negative half-cycle, the first op amp outputs zero volts (it can't go lower), so input V(-) is also 0V via the feedback. Input V(+) however is controlled by the input AC voltage and will fall to -2.5V at its minimum. Now most op amp specs say under Absolute Maximum Ratings: Input Voltage "Vee-0.3V to +32V" say). My question is, surely Vin at -2.5V is outside of the Absolute Maximum Rating of -0.3V? Also the Common Mode input at -1.25V is outside. So am I missing something? Any light anyone can shed on this will help a lot.

See
https://www.radiolocman.com/shem/schematics.html?di=161475
for the complete article.

ScotsDon
Page 19 of the LMC6482 data sheet addresses input voltages below the negative supply. A resistor in series with the input should limit the current to less than 5 mA and there is a caution about long term reliability. So, the input impedance of the device is very low under the negative input condition. This means the source driving the input signal will see a different load when the input is negative. This could be a source of gain error.

Also, the output of the first op amp will not go all the way to 0V when the input is negative. It will go to maybe 10mV of ground (since there is a very light load). This is greater than the input offset voltage of the first op amp. So, throw out the input offset voltage specification. The main offset voltage is from the output not driving all the way to 0V. However, all in all, a clever circuit.
 
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