# Analog Output circuit review

#### jonfarrugia

Joined Feb 19, 2010
71
I created an analog output circuit using a 3.3v pwm signal. The output is selectable using a jumper (P11) for either 0-5v or 0-10v.
Do it look ok to run with?

#### AnalogKid

Joined Aug 1, 2013
8,251
What is the purpose of R25?

ak

#### ScottWang

Joined Aug 23, 2012
6,860
What is the purpose of R25?

ak
What I think is that it was used to reducing the output current, but it will also affecting the output voltage when it is in series with the input resistance of the next circuit, the final output will be as a voltage divider output, it is depends on how high the input resistance of the next circuit

Joined Mar 10, 2018
3,785
Slew rate of the 2904 pretty slow at .5V / uS, is that OK with

R17, C7 will severely de-pulseify your waveform, change waveshape
to sine/heavily filtered exponential, that in line with what you are trying
to achieve ?

Regards, Dana.

#### jonfarrugia

Joined Feb 19, 2010
71
The purpose of R25 was to help protect the opamp from an overcurrent if it happens to get connected to ground. What else can I use to protect it? a poly fuse?

The whole idea of R17 and C7 was to convert the pwm pulse signal to an analog signal and then boost it to the desired voltage through the opamp

Joined Mar 10, 2018
3,785
You will want to put an R in series with V+ input to limit current when power is
turned off and C7 still charged, potentially its discharge thru power down OpAmp
could damage input. Based on your circuit maybe 40-50K would do.

Regards, Dana.

#### AnalogKid

Joined Aug 1, 2013
8,251
Sure you don't mean the non-inverting input?

Why so large?

ak

#### BobTPH

Joined Jun 5, 2013
2,198
Most opamps can handle a continuous short of their output.

Bob

Joined Mar 10, 2018
3,785
Sure you don't mean the non-inverting input?

Why so large?

ak
Yes, I thought the symbol was labeling NI as V+, now I see that is
power pin. Yes, series R with NI pin input.

Regards, Dana.

#### jonfarrugia

Joined Feb 19, 2010
71
Circuit Revised:

#### mvas

Joined Jun 19, 2017
538
Bypass Caps ?

#### jonfarrugia

Joined Feb 19, 2010
71
Is R50 really required? I havent seen another circuit with a resister of this size connected in this fasion

#### AnalogKid

Joined Aug 1, 2013
8,251
Depending on the opamp it could be a problem, but rarely. 50K is surprisingly large, and increases input bias current errors.

ak

#### jonfarrugia

Joined Feb 19, 2010
71
Would 10k be a better choice

Joined Mar 10, 2018
3,785
R50 was recommended to produce equivalent bias ofset Inv input
seeing.

But thats only 1/2 mV (room temp) , and for the application probably not relevant.

I withdraw the recommendation.

Regards, Dana.

#### jonfarrugia

Joined Feb 19, 2010
71
Then I will consider this circuit build able. Here is the final circuit:

Joined Mar 10, 2018
3,785
My apologies, I completely forgot about the issue with discharging the 10 uF
thru the input when supply is shutoff. Leave R19 in.

Regards, Dana.

#### jonfarrugia

Joined Feb 19, 2010
71
My apologies, I completely forgot about the issue with discharging the 10 uF
thru the input when supply is shutoff. Leave R19 in.

Regards, Dana.
use a 50k or 10k?