Low ADC Reference or an Op-Amp

[tom_pay]

Hi Everyone,

I have got a PIC which I want to measure a small signal voltage, ranging from 8mV to 180mV, with the ADC function. For simplicity I thought that I should just use a reference voltage of 200mV. However I was not sure weather or not an amplifier, and Vdd as the reference, would better suit the purpose. I would like the end product to be as accurate as possible, however it will receive regular calibration.

So, which would be better, a 200mV ADC reference voltage or an Op-Amp and Vdd as reference?

Thanks in advance,

Tom

 

[ericgibbs]

Hi Everyone,

I have got a PIC which I want to measure a small signal voltage, ranging from 8mV to 180mV, with the ADC function. For simplicity I thought that I should just use a reference voltage of 200mV. However I was not sure weather or not an amplifier, and Vdd as the reference, would better suit the purpose. I would like the end product to be as accurate as possible, however it will receive regular calibration.

So, which would be better, a 200mV ADC reference voltage or an Op-Amp and Vdd as reference?

Thanks in advance,

Tom

hi,
A regular PIC will not accept Vref's of less than +2V

 

[tom_pay]

Ok then, I guess an Op-Amp will do just fine then!!

How about an Op-Amp with a gain of about 19 with a reference of about 3.5v.

Also, what happens if the ADC input goes higher than the reference voltage? Does the magic smoke come out?

Tom

 

[ericgibbs]

Ok then, I guess an Op-Amp will do just fine then!!

How about an Op-Amp with a gain of about 19 with a reference of about 3.5v.

Also, what happens if the ADC input goes higher than the reference voltage? Does the magic smoke come out?

Tom

If you are operating the PIC at 5V. I would amplify so that the ADC input Vmax is 5V

Use a OPA with gain and offset so that the 8mV is nulled out to 0V and the 180mV becomes say 5V

 

[tom_pay]

Ok,

Though the op-amps wouldn't go to the full 5v. Or can you get ones that do?

At least this way there is no chance of going above the reference voltage of the ADC and releasing smoke!!

Tom

 

[ericgibbs]

Ok,

Though the op-amps wouldn't go to the full 5v. Or can you get ones that do?

At least this way there is no chance of going above the reference voltage of the ADC and releasing smoke!!

Tom

hi Tom.
The technique is choose an amplification factor that gives integer count value in the ADC.

eg: you input Span is 180mV -8 mV = 178mV

So if you chose a gain of 25, you would have a max Vout to the ADC of 4.45V
A low cost rail to rail OPA powered by +5V would easily output 4.45V [ and it would never go over 5v!]

This would give 910 counts maximum for 178mV * 25 =4.45V

 

[tom_pay]

Ive had a look at some Op-Amp data sheets an I cant find their output voltage range. Mainly the LM224's (I have a few as surplus). What should I look under to find its maximum output voltage?

Is it possible to have an input of 8mV and an output of 0V with a single op-amp? I will be a bit short of board space.

Thanks

Tom

 

[Nigel Goodwin]

Ive had a look at some Op-Amp data sheets an I cant find their output voltage range. Mainly the LM224's (I have a few as surplus). What should I look under to find its maximum output voltage?

Is it possible to have an input of 8mV and an output of 0V with a single op-amp? I will be a bit short of board space.

Check my PIC tutorials, in particular the hardware for the analogue one - you're not the first to want to do this.

 

[tom_pay]

Thanks Nigel, they are very good, unfortunately they're in ASM so I used Ian's translations.

Just out of curiosity; what happens if the ADC input exceeds the ADC reference? Does the magic smoke come out?

Thanks heaps,

 

[Nigel Goodwin]

Thanks Nigel, they are very good, unfortunately they're in ASM so I used Ian's translations.

The hardware isn't in ASM!

Just out of curiosity; what happens if the ADC input exceeds the ADC reference? Does the magic smoke come out?

Nothing nasty, it will just not give an accurate reading - it 'may' just read 1023, I've never actually tried it.

 

[tom_pay]

The hardware isn't in ASM!

He, He I suppose so!!

Im not game enough to try, I hope nothing bad happens!!

 

[ericgibbs]

He, He I suppose so!!

Im not game enough to try, I hope nothing bad happens!!

What exactly are you worried about.??

If your supply to the OPA is greater than 5V, then use a series 270R resistor in series with the ADC input pin and a 5V1 zener diode from the pin to 0V.

 

[Nigel Goodwin]

What exactly are you worried about.??

If your supply to the OPA is greater than 5V, then use a series 270R resistor in series with the ADC input pin and a 5V1 zener diode from the pin to 0V.

You don't need a zener, there's a clipping diode on the input already - you just need a current limiting resistor (as used in my tutorial hardware - and the reason why it's there), but 270 ohm is a bit on the low side.