Hi

...

IOUT_ AD5420 and IOUT_ AD5420_R are two terminal of resistor in a 4-20mA current loop where I'm sensing. As HART signals also flows through my 4-20mA loop, Is there any dependency while selecting filter to this instumentation amplifier with respect to frequency. HART signals at loop(FSK signal) will be at frequency 1.2KHz and 2.2KHz. Please help me out.

Regards

Nidhi P Shetty

Parents
Children
• Hi Nidhi P. Shetty

I would think that you'll want this 1.2kHZ to 2.2kHz signal be detected on the AD8226.

Checking out our Designer's guide to instrumentation Amplifiers, page 5-14 shows the general rules in selecting the RC input filter. For the resistor values, looking at the AD5420 it can only handle about 1.2kohms load, I would suggest you start there and adjust Cc and CD accordingly (Differential BW should be 10 times higher than the desired frequency which in your case is 2.2kHz signal).

Page 5-12 to 5-23 provides a lot of additional information on RFI filters.
Let me know if this answers your question or if you have more concerns Nidhi P. Shetty.

Best Regards,
Dann

• My concern is that AD8226 should not allow 1.2Khz and 2.2KHz. I have a parallel path for 1.2KHz and 2.2KHz. How I can avoid 1.2KHz and 2.2KHz at AD8226

• Your statement is corrected as per my query "I would think that you'll want this 1.2kHZ to 2.2kHz signal be detected on the AD8226." is changed to " I don't want my AD8226 circuit to detect 1.2Khz and 2.2KHz. Is it possible?. Please help me out ASAP.

• Hi Nidhi P. Shetty,

In that case you need to lower the differential bandwidth of the filter to cutoff the signals from 1.2kHz and higher. The Differential BW and Common-mode bandwidth is given by the formula (from AD8226 datasheet):

You can still start with R = 1kohms as this is close to the maximum resistive load of AD5420 (which is 1.2kohm). With that you can set the CD to 80nF and CC to 8nF. you'll have a differential BW of around 950Hz, let me know if this works for you. Also, what's the configuration of the Iout of your AD5420?
Apologies for the delayed response.

Best Regards,
Dann

• AD5420 IOUT will be flowing in a loop along with it's HART. In between that I'm sensing across a external resistor in loop using AD8226 . AD8226 output is given to ADC to provide a feedback and automatically correct error if any. So my concern is that HART should not get effected because of this sensing. As I'm sensing voltage anyways 0-20mA/4-20mA current will not get effected. But I've concern regarding HART.As it is a separate path from that of load how can I directly decide that the resistor value can be upto load value.

Similar configuration is there with AD5421 as well in my design.