# Setting up circuit with AD8223

I have several questions about using this instrumentation amplifier.  I am using it to amplify a signal that varies from 0-30 mV to 0-3 mV.

The input signal and the supply voltages may have some RFI on them, although I have no idea what the frequency would be.  So I am looking at Figure 34 on page 15 of the datasheet, where a suggested low pass filter is posted.  Could I use the exact values of series resistors and capacitors on the input side and a.) not affect my input voltages greatly, considering how low they are and b.) remove RFI with these values?

Also, I see that this figure has multiple capacitors on the supply side.  What are these for?  Why are there two on each side?  Why does one of the capacitors on each side have a curved line and a plus sign?

Thanks.

So for example, I calculate that if I use CD = 10 nF and CC = 100 nF and R = 7580, my differential frequency cutoff is 100 Hz and my common mode frequency cutoff is 2100 Hz.

Do those seem like reasonable values to use for the capacitance and resistance to target these cutoff frequencies?  The ratio of capacitor size to resistor size seems arbitrary.  I have not designed a circuit like this before, I don't have a good feel for what I need to be aiming for.  My main concern is to not introduce an overall error to the voltage I am reading, especially since the input voltage is so small: small changes have a big effect.

Thanks again.

• Usually, the frequency of my input signal is not set to a constant: the value is set by a manual control, and it is adjusted very slowly.

However, we also would like to operate it in pulsed operation, possibly up to 150 kHz.  However, looking at the bottom of page 3 of the datasheet for AD8226, it looks like the dynamic response is only 20 kHz at a gain of 100: I assume that this means that it can resolve a frequency of 20 kHz at that gain.  I need to run this at 100 gain, so that would not be enough.  Unless you have another amplifier like this that can resolve a higher frequency.

Also, wouldn't it be a voltage divider of 7k and 0.8 Gohm, rather than 100 meg?  The datasheet says the input impedance is 0.8 Gohm for common-mode, and 0.4 Gohm for differential.

• First, I would use the AD8226 which has the newer, improved pinout, lower supply current, lower noise,

and higher bw.

The equations are given for the R/C values.  It depends on your highest signal frequency.  If you are looking

at 10 Hz signals, then you can use a filter cutoff of 100 Hz, or even 30-40 Hz.  For a 1 kHz desired signal,

you would use higher cutoffs.

For any IC, the amount of bypassing depends on how clean your supplies are.  The small caps are good

for high frequency;  the large caps are good for low frequency and heavy load swings.

Harry

• What frequency is your input signal??

The input impedance of InAmps is very high, so the voltage divider of 7k and 100meg? is not much.

Harry

• This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

Thank you,