AMC1200: capacitor at input pin

Part Number: AMC1200

Hi,

in AMC1200 datasheet Figure 32. Typical Application Diagram, there are C3 from VINN to GND and C4 from VINP to GND, and it's said to be optional.

Optionally, C3 and C4 can be used to reduce charge dumping from the inputs. In this case, take care when choosing the quality of these capacitors; mismatch in values of these capacitors leads to a common mode error at the modulator input. If implemented, TI recommends using NP0 capacitors for C2 , C3 and C4.

I don't understand what does charge dump mean here. Does it mean that in Sample and Hold stage, the internal capacitor is charged through C2 and C3? But we've already have CINP and CINN=3pF here, which should have the same effect as C2, C3. 

And the customer also asks how we do the sample, is it that we get (VINP-VINN) analog signal and then do the digitization, or we digitize VINP and VINN seperately and then do the subtraction with the digitized value?

I guess it's the first case, during the sample and hold stage is: CIND is charged to VINP-VINN, and then this analog signal is digitized, right?

  • Hi Howard,

    this is just a charge kick-back filter. During the internal switchings of AMC1200 current spikes are emitted at the inputs and benefit from the current path back to the AMC1200 via C3 and C4. This is no rocket science but just very well known and very good design practise Relaxed

    Kai

  • kai,

    what does kick-back mean? Why are there current spikes at the input?

    And the second question is: is it that we get (VINP-VINN) analog signal and then do the digitization, or we digitize VINP and VINN seperately and then do the subtraction with the digitized value?

  • Hi Howard, 

    As Kai said (thank you Kai!) these capacitors can be added to help reduce kick-back. This is referring to the behavior of the analog input signal when the load (S1) is connected and disconnected. These capacitors would help the analog input settle faster and/or be disrupted less. As the datasheet says, they are optional and not required. 

    For the second question, the modulator is differential. The analog input signal is subtracted at the input of the modulator (VINP-VINN), digitized by the modulator and then sent across the isolation barrier as 1's and 0's via discharging capacitors. The burst of energy from the capacitors are recovered into 1's and 0's, passed through a digital filter and buffered, producing an analog output.