ADS1675: ADS1675

Hello Robert,

Welcome to the TI E2E community.

The differential inputs AINP and AINN are a switched capacitor input, and have an equivalent input impedance of about 2.5kOhm with fCLK=32MHz.  The only way to reduce this loading effect is to reduce the frequency of fCLK.

The impedance can be calculated by:

Zeff = 2.5kOhm*(32Mhz/fCLK), where fCLK is the input clock to the ADC

It is suggested to use a fully differential input amplifier similar to Figure 54 in the datasheet to drive the ADC inputs.

Regards,
Keith Nicholas
Precision ADC Applications

Hi   Keith

     When I  slow down the fclk    the  current between the AIN and AIP   will decrease but still have, and could we have some other method to solve this problem? We  already have used ADS1274, And it doesn’t have such problem,  thanks!

Hello Robert,

The ADS1274 has a similar input stage, but the equivalent input impedance is higher (lower input current) compared to ADS1675.

Since the input current depends on the input impedance, you can reduce the maximum differential input voltage on the ADS1675 to limit the maximum amount of input current, in addition to reducing the frequency of fCLK.  For example, with fCLK=8MHz, the input impedance will increase to 10kOhm.  If you limit the maximum input differential voltage to +/-1V, then the maximum input current will be AINP-current=1/10k= 100 microamp.

1. What output data rate do you need in your system?

2. What is the frequency of CLK?

3. How are you driving the ADC inputs, AINP and AINN?

Regards,
Keith

HI

   My purpose is used to measure the  microamp, if the input current is 100 microamp, how can I use the AD to measure the microamp currrrent?