Chopper stabilised op-amps - is there a mystery to the noise?

Since the TI data sheet carefully avoids mentioning high frequency noise, they probably have something to hide. But a delta-sigma ADC mostly ignores noise, so you are probably OK. But an RC is cheap insurance.

We use delta-sigma ADCs directly on thermocouples, no preamp.

A bypass on the input is the ticket. Many chopamps produce nasty current spikes out their inputs ("kickout" to old-timers).

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

John - what delta-sigma adc and what thermocouples do you use? That should serve as a way around a collection of packages.

Hul

John Lark> > >I am using the TLV2333 device; actually the cheaper Philips version of

ADS1247/8 are nice parts. They have a front-end mux and a PGA with gain from 1 to 128. 24 bits and a burnout current source, and a good internal reference.

We short the inputs now and then to take out any input offset. You can use the internal mux to do that. We don't use the complex internal calibration stuff.

They can also do ratiometric resistance measurement to insane accuracy. We generally use a platinum RTD as the reference junction temp sensor. The ref junction sensor is usually the worst part of a thermocouple system.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

John Larkin wrote

The ADS1118 goes down to 200mV FS which is OK for the higher output TCs but not for the low output ones used in hi temp apps. Well, it will obviously work, but it won't be 16 bit :) More like 10-12 bit.

John uses ADS1247, which has a PGA gain of 128, 16x higher. They claim a 48nV noise level at that gain. And he uses the 2nd channel to measure a shorted input for offset correction.

--
 Thanks, 
    - Win

We also use one per board to pick up platinum RTDs as the reference junction temps. We put a good Susumu resistor in series with the RTD, measure both voltages, and do the math.

We have a local RTD on board, and also allow the user to have his own RTD in a remote reference junction box.

Here's one that we offer:

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

How do you deal with wire resistance for distant remote boxes? Three-terminal or four? It'd be interesting to see your wiring setup details, with the ADC's multiplexing inputs.

--
 Thanks, 
    - Win

4-wire of course.

Here's a schematic that I have handy. It combines BIST with acquiring three RTDs.

This is an old rev. We discovered that the AD7793 is flakey and replaced it with a TI part.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

What is the TI part?

ADS1246 and ADS1247 in various places. Works great.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Thanks. If you have the time, could you describe what made the AD7793 flakey?

It has in internal 24-bit shift register. But nothing initializes the

24-bit counter after powerup. De-asserting chip select doesn't initialize it. So if you ever miss one count/clock, the data is forever trashed.

And when it gets cold, it misses.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

These are Major design flaws. So now we have to put any new device through major qualification testing!!

ADI used to design great analog stuff with bizarre and broken SPI interfaces. One serial ADC had a bit in a control register that determined if the SPI interface accepted data on the rising or falling edge of the SPI clock.

Reminds me of the old days in the USSR, when there were no road maps. If you don't know where you are, you shouldn't be there.

Maybe they are better now. Both.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

GPS and Google Earth have spoiled the map-hiding game.

--

-TV

Thanks John. As you say, the local reference sensor is a problem. Might use the adc to measue a 1n914 or similiarly heavily used diode and sort the diodes as they come in the door.

Hul

snipped-for-privacy@highlandsniptechnology.com wrote:

That sounds like a hassle. We use a surface-mount 1206 platinum RTD. We put it in series with a good Susumu resistor and use the delta-sigma ADC to measure the voltage across each. A bit of math gets the RTD resistance, and a bit more gets the temperature. It's easy and super accurate.

Wow, sounds interesting, p/n?

--
 Thanks, 
    - Win