AD8421

Semiconductor Integrated Circuits (ICs)
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1

Hi,

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?.

2

Greetings,

The purpose of the REF pin on an instrumentation amplifier is to set the DC bias level of the output. Within limits described by the absolute maximum ratings, you can connect it however you like.

The arrangement in figure 72 is designed to interface a ±250mV input to an ADC that accepts a 0~5V input. Doing that requires creating a 2.5V offset, which the OP1177 is used to help create. If your signal is already properly scaled to the input range of your ADC, then a simple buffer amplifier with no offset should be sufficient.

3

Hi Rick,

So, I will connect the REF pin to GND, Inverting pin to GND. non-inverting pin to 5V as input voltage to the amplifier. Thus, Is it work as a buffer amplifier?

4

If the result meets your design requirements, then yes that would be a valid option.