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ad7793 weirdness

I'm prototyping an RTD based temperature sensor using the analog devices AD7793 24-bit ADC. I seem to be seeing some unexpected results when feeding a small fixed voltage (340 millivolts) and varying the gain of the AD7793. Reference voltage is fixed at approx

2.26 volts.

If I set the ADC for unity gain, I see a reading of 2,531,837... If I set the ADC for 2x gain, I see a reading of 5,071,765... Which is reasonable... Now if I set ADC for 4x gain, I get an unusual reading of

6,116,603... Not double the 2x reading that one would expect. If I then set the ADC for 8x gain, I see a reading of 12,050,792... Which is about double the 4x reading, but not 8x the unity reading that one would expect.

I've tried doing internal fullscale and zero calibrations without any real change.

I did find a line in the datasheet which worries me: "When the in-amp is active (gain >= 4), the common-mode voltage (AIN(+) + AIN(-))/2 must be greater than or equal to 0.5 V." Surely that is a mistake. That requirement would render the gain function nearly worthless since small input values is when you are going to want to use the higher gains.

Can anyone help me figure out what I'm overlooking?

thanks.

Reply to
jakeb221
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It might not be a mistake: It's quite common to have some small signal sitting on top of "some volts" of common mode voltage... and indeed, you can arrange this yourself by just adding some bias resistors and coupling capacitors if you have an AC signal (probably not for most RTD usages?) or using op-amps if not (albeit with op-amps the analysis of what it does to very tiny signals can take some effort).

I'd ask ADI for certain...

Reply to
Joel Koltner

"The AD7792/AD7793 can be operated in unbuffered mode only when the gain equals 1 or 2. At higher gains, the buffer is automatically enabled. The absolute input voltage range in buffered mode is restricted to a range between GND + 100 mV and AVDD ? 100 mV. When the gain is set to 4 or higher, the in-amp is enabled. The absolute input voltage range when the in-amp is active is restricted to a range between GND + 300 mV and AVDD - 1.1 V. Take care in setting up the common-mode voltage so that these limits are not exceeded..."

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

It means that for higher gain settings, you must bias the inputs such that the average is above 0.5V. For instance, you could have 1.000V on AIN- and 1.340V on AIN+.

For RTD measurement, check fig. 21 of the datasheet. The Rref value must be high enough such that (Iout1 + Iout2) * Rref > 0.5 V

Reply to
Arlet Ottens

Anything *bad* in a data sheet can generally be taken as true..

Since generally you're running RTDs with constant current sources (one source for 2-wire or 4-wire and two for 3-wire), and since RTDs are generally isolated, a simple resistor to circuit ground, preferably with a bypass, should suffice.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

By the way, if you're not using this 3-wire RTD configuration, you may find the built-in bias voltage generator useful. See bits 14/15 of the configuration register.

Reply to
Arlet Ottens

Thanks for all the quick responses... I am using fig 21 for the RTD, but during this experimentation/testing, I just drove the excitation current thru the reference resistor, grounded AIN- and connected to AIN

  • to the small variable voltage source.

So if I follow fig 21, using a 4.99K reference resistor, 210 uA excitation current, and 100 ohm RTD, then the higher gains should work fine?

Reply to
jakeb221

I don't see how that would help, unless he has a completely isolated current source, which seems unlikely.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Yes. I'm using the device in a very similar way, and it works fine.

Reply to
Arlet Ottens

For a 2-wire rtd, make a voltage divider from any old Vref, with a reference resistor and the rtd. Digitize the voltage across both and do the math. Tweaks can do 3-wire and keep the common-mode voltage up.

But with a 24-bit adc, you can turn off the buffer and let one end of the string be ground.

John

Reply to
John Larkin

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