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ADC protection - low noise

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kolpe

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Hello,
I'm using a ADS8344 16bit ADC converter.
I have recently added simple clamping diodes (BAT85) and a 33Ohm series resistor to protect the inputs. This has introduced quite a bit of noise.

Can anyone suggest low noise diodes or a different approach?
(I'd rather avoid an additional OpAmp)

Thanks
 
For a 16-bit ADC to be useful you need things like really quiet power supplies, really accurate and stable voltage references, ground planes, and a whole lot of calibration. So a lot of times it's a bit pointless to use a 16-bit ADC. Did you just pick it because it was the highest resolution you could find? Most 16-bit ADCs aren't even accurate to 16-bits (so a 14-bit ADC might do just the same and be able to sample faster too). If you used a 12-bit ADC it would sample even faster. It takes quite a lot to squeeze out the full accuracy of a 16-bit ADC.

What are you using it for? Do you even need 16-bits? Do you even need a clamping diode and a current limiting resistor at the input? Because at 16-bits you want to add as little as possible since everything will introduce noise.

And what makes you think most of the noise is coming from the diode? If it's clamping it's probably connected between input and +V right? I would think most of the noise comes from the resistor sitting in series with the input.
 
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Thanks for your suggestions.

dknguyen said:
Bypass caps? Ground rings? Ground planes? Better regulator? Quiet regulator? Filtered regulator?

I think I did most of that when designing towards 16bit. Took a while but I now get very little noise. That is, until I add the clamps.

Do you even need 16-bits?

well, yes...

Why do you have a resistor anyways? Why didn't you just connect the diode (and maybe resistor) between the input and +V? How did you connect it?

Regarding the resistor I took that suggestion from the ADS8344EVM. Clamping is towards V+ and Ground at the ADC input.

Anything you add into the circuit is going to introduce noise and at 16-bits you just lose that accuracy. Not much you can do about it except to remove the clamps.

Well, I feared as much.
Just thought there might be some clamps that are somewhat better than the BAT85.

Do the inputs even need clamping and a protection resistor?

Not until recently, but now there's been some situations where it is required.

And why do you want to avoid an op-amp?

I don't want to add a split-supply. And I'm not really up to date if there's ways to achieve full-range (0-5V) with a single-supply setup.

Thanks.

Oops, just saw that you've edited your post. I will try to remove the resistor and see how much is the diodes and how much the resistor.
 
Cover the diodes because they are sensitive to light. A 1N4148 is a photo-diode. Your BAT85 diodes are also in a glass case. Is it clear glass?
They are Schottkey so their leakage current is high.
Keep them away from anything that is warm.
 
Diodes are inherntly noisy in fact alll semis are but things like diodes make great AM radio receivers. Also did you connect the ground diode to the analog ground (should be ok) or the digital(BAD) ground?

I prefer using an PNP transistor to clamp voltage above VCC connected as follows

https://img128.imageshack.us/my.php?image=alternateclampeo5.jpg

when input goes above VCC+Vbe transistor turns on and clamps it until signal returns below VCC+Vbe

If that still isnt clean enough try adding a resistor to the base .
 
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