Hello, I'm making a loadcell amplifier board. Input from -18mVDC to +18mVDC. Output +-2.5V. Input signal frequency from 0 to 14Hz. I have made a design as the attach. The total gain changes from about 140 to 50 according to real condition.
- First, I don't know why the output is very fluctuated and gaining is wrong.
- Second, I intend changing the low-pass filter to pre-amp + filter. The IA will became to post-amp.
Anyone help me what is the wrong in my design and how to solve it.
I am sorry once again for the delay.
Assuming that the AD8295 in-amp inputs and outputs are within a working range, the Rg pin will be a small diode drop ~0.5V below the corresponding input. So with both inputs at a voltage near 0V, both Rg pins will be at about –0.5V. You can observe this with a DVM if you disconnect the digipot from the Rg pins. The internal feedback conditions for the in-amp ensure that this voltage drop is constant, which imposes your differential input voltage across Rg. See the AD8221 data sheet for a more detailed drawing of a similar instrumentation amplifier.
The AD8231 input voltage range is relative to the supply, no matter whether it is a single supply or a dual supply. Looking at the +5V single supply table, you could rewrite the input range as -Vs + 0.05V to +Vs - 0.05V. ±2.5V is the same voltage span as +5V, so it will translate directly as -2.45V to +2.45V.
To check all of the in-amp headroom limits for the AD8231, including input voltage range, output voltage range, and internal nodes, you can have a look at our in amp tool: Instrumentation Amplifier Tool - Common Mode vs. Output Voltage (Diamond Plot)
I hope this helps.