Need a review of a Front-End Circuit for an A2D Converter

[ProEngineer]

I have a project that is basically a Spectrum Analyzer that needs to accept in input from 5 to 600VAC (RMS). The front-end needs to be auto-ranging and include a low-pass filter (DC to 3KHz). The attached file is my proposed circuit. Before I commit it to a PCB, I'd appreciate it if some of you review it and comment.

Some Notes:
- RL1 through RL8 are relay contacts; the coils will be driven by a uP
- The Diodes are there to clamp the input to the Op Amp
- The sum of R1 through R8 will be 500K, giving me a 1M input impedence

 

[karesz]

Hi,
In my opinion is some logically wrong: for me are the ranges 5V...1KV rotated!
I mean- for 1KV on the input must RL1 not between R10&R1, but correcter R7&R8 be; and so on...
These because you need the highest attenuation with 1KV on the input.
K.

 

[ProEngineer]

Yep, you're absolutely right about that!

 

[karesz]

Your welcome & tnx!
Good luck!
K.

 

[ProEngineer]

Nobody else have any comments?

 

[keith1200rs]

Re: Need a review of a Front-End Circuit for an A2D Converte

Nobody else have any comments?

You could save on the number of relays if you wanted by having two stages e.g. 3 relays followed by 3 more relays.

You filter looks strange, but maybe it's just a configuration I am not familiar with. Normally I would expect C2 to be on the opamp input, R11 to be a capacitor and C1 to be a resistor.

Keith.

 

[ProEngineer]

Re: Need a review of a Front-End Circuit for an A2D Converte

You could save on the number of relays if you wanted by having two stages e.g. 3 relays followed by 3 more relays.

Yes you are right, but I'm driving the relays using a 3-to-8 line decoder to reduce the pin count on the uP.

You filter looks strange, but maybe it's just a configuration I am not familiar with. Normally I would expect C2 to be on the opamp input, R11 to be a capacitor and C1 to be a resistor.

My circuit is a low-pass filter, I believe what you are describing is a band-pass configuration.

 

[keith1200rs]

Re: Need a review of a Front-End Circuit for an A2D Converte

You filter looks strange, but maybe it's just a configuration I am not familiar with. Normally I would expect C2 to be on the opamp input, R11 to be a capacitor and C1 to be a resistor.

My circuit is a low-pass filter, I believe what you are describing is a band-pass configuration.

No, I am definitely thinking of a low pass filter design, but maybe it is just different to yours:

https://en.wikipedia.org/wiki/File:Sallen-Key_Lowpass_General.svg

Keith.

 

[ProEngineer]

Re: Need a review of a Front-End Circuit for an A2D Converte

No, I am definitely thinking of a low pass filter design, but maybe it is just different to yours: https://en.wikipedia.org/wiki/File:Sallen-Key_Lowpass_General.svg
Keith.

Ok, I see...yours is a Sallen-Key topology while mine is a Multiple-Feedback. I don't know if one is better than the other for this application. Maybe one of the Analog Experts will chime in here.

 

[keith1200rs]

Re: Need a review of a Front-End Circuit for an A2D Converte

Ok, I see...yours is a Sallen-Key topology while mine is a Multiple-Feedback. I don't know if one is better than the other for this application. Maybe one of the Analog Experts will chime in here.

I think the fact that Sallen-Key is non-inverting and Multiple Feedback is inverting is the main difference. Both can have gain.

Keith.