Clamping circuit for PIC A/D

[Rajinder]

Hi,
I was wondering if someone could help me.
I have a 60V pk to pk signal, which has a current of 200uA, so not much.
I need to clamp this so I can feed this to an A/D input of the PIC microcontroller (PIC18F). The PIC can have its A/D from 0-5V

My solution is to use a resistor and zener diode (3V6) to clamp the voltage to around 3.6V. The feed is to the resistor and the other end connects to the Zener cathode. The Zener anode connects to 0V.

I am not sure how to get rid of the -ve of the signal, I think the zener will clamp the negative supply to -0.7V. Could I connect the anode to 1V supply and then it would be -0.3V?

How can I get this to be 0V to 5V range? Will the zener work at such a low current i.e. 200uA.
I am not sure of the A/D apart from it has 10K source impedance maximum.

I have been told that a FET and zener diode will be a better option, I am not sure how this works , I presume the FET is used to increase the input impedance to the circuit? Can anyone advise?

I look forward to any help
Thanks in advance
Raj

 

[Minder]

So ~20v rms if it is a constant, at what frequency? What do you intend to do with the signal?
A small ferrite transformer may do it depending on the frequency and small bridge rect..
M.

 

[Rajinder]

Sorry frequency is 4Khz it is from the feedback pin of a piezo. I thought the easiest option was to clamp this using a zener but was unsure what would happen to the A/D input of the PIC. The signal detects if the piezo is working or not.

 

[Minder]

If this is just a go/no-go situation, a 2n7000 and an opto isolator may do it.
M.

 

[Rajinder]

Yes it is just to see if the piece is working bit it delivers 60V pk to pk..but I am not sure on your circuitvto reduce this to 0 to 5v.

 

[Minder]

A schottky bridge followed by a 2n7000 with a input to a high speed opto with a output pulled up to 5v for input to the Picmicro.
M.

 

[Rajinder]

Thanks. Is my zener method not good

 

[Minder]

Trying to keep a high impedance load to the source. 200μa!
M.

 

[Rajinder]

Thanks for your help. Could you show me a circuit diagram please.

My idea was a zener to clamp the voltage say to 3V9 then use a GET as source follower. But this is info that I am not sure how to implement 100%.

I'
A bit confused you mention high impedance
Load to source. If the PIC had 10k source input impedance on AD pin. Then shouldn' source be lower. I'm a bit of a novice so any help understanding this would be helpful too.
I appreciate all the help.

 

[metalcore]

Depend upon the frequency level.

 

[Rajinder]

Please could you explain.