Component connections

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Joined Nov 28, 2019

Im planning to use the AD8421 component to drive the ADC. So, in this case, where do I need to connect the reference pin of the AD8421?. According to the datasheet, in the page 24 saying that the reference pin is connected to GND. And, in the page 28 (Figure 72) saying that reference pin connected to (the voltage) the output pin of the external amplifier. But, I simulated the circuit in both ways by connecting GND and by using the external amplifier (OP1177), I got two different result. I'm applying 5V to +IN (non-inverting terminal) and I want output should be 5V, this result yields when the REF pin connected to GND. When I simulate the circuit according the Figure 72, I got the appox. 7.5V (5V from input terminal + 2.5 from output of external amplifier (OP1177)) at the output of the AD8421, so the figure 72 connections not useful for my application. But, the figure 72 is designed for ADC drive. So, I'm not understanding how to design the circuit, Shall I connect the REF pin to GND?.


Joined Jan 30, 2016
The REF input of the AD8421 allows you to set the offset of the output of the AD8421 to suit the ADC active range. If your ADC only reads 0 - 5v and the input to your AD8421 is 0 - 5v then REF = GND is what you want. If your input was, say, +/-2.5v and your ADC was 0 - 5v then putting REF = 2.5v would lift the AD8421 output by 2.5v.

Example application circuits in datasheets are to show you how to use different features of the device. Its up to the designer to understand how to apply them to their specific needs. You have to read and understand the text not just look at the pictures. If you do that you will see fig72 applies to a very specific set of circumstances (input +/-250mV,input gain x10, output +/-2.5v centred on +2.5v to drive ADC 0-5v).

What ADC are you using and what are you measuring?
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