AD621 drift with temperature

Matt,

Thank you for the reply and for the information.

I am using the AD621SQ package and the bridge resistance is 100K. I will change the bridge resistance down to match the bridge resistance of the pressure transducer I am planning on using (1.5K) and then do the other checks you recommend.

Thanks again.

Your output is -113 mV at ambient and -95 mV at 125 degrees.  With the AD621 at G=100 this means the differential input should be -1130 uV at ambient and -950 uV at 125 degrees.   This is a 180 uV change.  If we assume your ambient is 25 degrees, then we have 180 uV/100 degrees = 1.8 uV/degree C change.  For the A grade version of the AD621, we specify 1 uV/degree C from -40 to 85 degrees.

Let's check if a change in power supply voltage could cause the change.  The minimum PSRR for the AD621 is 110 dB in G=100.  We only specify this at room temperature, but I am doubtful it will change enough over temperature to drop much below 110 dB.  If we assume 110 dB over temperature, this is roughly 3 uV/V.  This means it would take a 60V change in power supply voltage to get the 180 uV change you are seeing - I think we can rule this one out.

Some things to check:

1)  Ground the inputs of the AD621 as close as possible to the input pins and check the drift.  This will totally eliminate any errors that might come in from the resistor bridge, and help determine whether it is an issue with the electronics or the input connections.

2)  Check at ambient, 85, and 125.  While I would expect most AD621's to have fairly good offset drift performance up to 125, we only guarantee up to 85, and that might be the issue (unless you are using the much more expensive S grade).

3)  I assume your bridge resistances are fairly reasonable (say <10 kohm).  If you were using big values for the resistors, bias current drift might come into play.

If find through the above checks the AD621 is not performing as desired from 85 to 125, another part you might consider is the AD8231.  It has excellent offset drift performance from -40 to 125.  However you would have to change your supply scheme in order to use it.  It is a single supply part.

Hope this helps.

Matt

Ok, I changed the bridge resistance to 1.5K, still had the same drift. So I shorted the input pins together and heated the AD621.

At 70degF the output pin read .3mV and at 250degF the output pin read 2.3mV, I would expect the max drift to be 99uV. Any ideas what might be wrong?

70°F -> 21.1°C

250°F -> 121.1°C

Max average TC of AD621S is 1.0 uV/°C.  So across your 100°C difference I would expect (like you) to have a max drift of 100 uV.

Typ average TC of AD621S is 0.3 uV/°C.  So across your 100°C difference I would expect typical drift to be about 30 uV.

However these offset numbers above are referrred to input (RTI).  So with a gain of 100, referred to output (RTO) this numbers become:

Max:  10 mV

Typ: 3 mV

The drift you see at the output is 2 mV, so it sounds like your AD621 is performing pretty close to the typical specification.

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I'm not sure why you are still getting drift issues with a 1.5k bridge.  One thing you could try to gain insight:  connect a couple of thin film 750 ohm  resistors (outside the oven) in series with the inputs, which are then connected to ground.  This source resistance should look exactly the same as the 1.5 kohm bridge to the AD621.  750 ohms is basically nothing for the AD621's low bias currents, so I wouldn't expect any issues.   If you do see some issues it might be noise is coupling into the input leads - the way to typically fix this is to use an RFI filter.  See page 18 of the AD8228 datasheet for how to do this. 

Matt

Mat,

     Thanks for the response and the info. I didn't think about applying the gain to the input offset drift  - that makes sense. I will try the 750ohm resistors and see what happens. 

     Thanks again for your time.