Choosing output resistor of AD623

Thread Starter

Domophone

Joined Sep 10, 2013
65
Hi, i'm trying to use an AD623 in LTSpice.

The setup: It is a differential amplifier, that i've got set with a gain of 1. ie: I apply a voltage of 100mV on the differential input, I expect an output of 100mV. The purpose of this amplifier is to remove a common mode voltage, and lower the impedance of the signal. I have the amp set up on a single supply with both input and output referenced to ground right now. I've narrowed my problem down to what seems like the output resistor. I've cut it down to as simple as i can make it, and do not understand the output i'm getting.
upload_2018-11-6_11-0-25.png

The problem: the simulation outputs 25.5V with a 10k load resistor, or a 30k load resistor. The output tracks the input, as it should, if the load resistor is 15k. The datasheet mentions on page 23, "The AD623 is designed to drive loads of 10 kΩ or greater." but i can find no other limitations that I'm infringing on. Output reference (REF), according to the datasheet, can swing from rail to rail.

10k Load... upload_2018-11-6_11-1-27.png

15k Load...upload_2018-11-6_11-5-43.png

30k Load...upload_2018-11-6_11-9-3.png



Resources:
The model is available here: https://www.analog.com/en/license/spice-models?mediaPath=media/en/simulation-models/spice-models/ad623.cir&modelType=spice-models

Datasheet here:https://www.analog.com/media/en/technical-documentation/data-sheets/ad623.pdf

I've attached my spice model.

Any advice/explanation seriously appreciated!
 

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Thread Starter

Domophone

Joined Sep 10, 2013
65
EDIT: ASC file now matches image.

Sorry, I had changed it to use a step function to change load resistor value. will re-upload, but i'm currently updating LTSpice so it will be a few minutes. Hopefully they have a native model now (haven't updated in about a year) that analog has aquired linear.
 
Last edited:

Thread Starter

Domophone

Joined Sep 10, 2013
65
ya i figured there was no built in boost converter ;). i just figured the model was spitting out nonsense, but i don't understand why, or what limitations i've exceeded.
 
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