ADS1298: how to increase input impedence

Hello Alex,
Thanks for your reply~
Do I have any other ways to increse input impedance?
Now we are facing this situation:when the input signal is 10Hz，their is almost no output attenuation;but when the input signal is 100Hz，output attenuation is much more~

Hi Fawn,

I am happy to help!

Adding an additional buffer circuit is possible, but I do not understand why you need this when the input impedance of the device is already much higher than what you require.

What is the cutoff frequency of your anti-aliasing RC filter?

Hello Alex,
Maybe the input impedance now is not as high as the datasheet said. Sorry that I have not tested it now~

May I know if the RC filter related to the input impedance?

Hello Alex,
I found that when it's Current source lead-off detection,the DC input impedance typical value is 500MΩ，however,the DC input impedance typical value is 10MΩ when in Pullup resistor lead-off detection mode,maybe I'll check the Lead-Off Control Register~

But if it's in Current source lead-off detection mode , the input impedance is not as higher as described in the datasheet,what else shoul I do?

Thanks very much for your kindly help~

Hi Fawn,

The input impedance will not vary by a large amount from device to device. There may be a slight variation in input impedance, but it will not be orders of magnitude unless there is a defect of some sort. Devices with defects should be removed from the production line before being purchased by the customer.

If your customer is using current source lead-off detection and requires higher input impedance than I suggest looking into a high impedance unity gain op-amp to buffer the inputs of the device.

The RC filter is not implemented for increasing the input impedance. The RC filter is used to attenuate the high frequency signals that alias back into the band of interest being measured. Your RC filter cutoff frequency could be too low, which would explain why your signals at 100Hz are being attenuated and the signals at 10Hz are passing through as expected.

Dear Alex,
I've learned a lot from you~ thanks a lot~

Another question,what op-amp will the best choice(price/perfermance) I can choose?