That is my circuit: enter image description here

Input: 400V DC

U2_1: LTC1285 (ADC) http://www.farnell.com/datasheets/1575635.pdf

U2_2: IL261 (digital isolator) http://www.nve.com/Downloads/il260-1.pdf

The circuit behavior is:

1: resistive partitor with high precision resistor to decrease the input voltage.

2: zener diode protect from eventually overvoltage.

Now the question:

In point 3 is useful to use a low pass filter for clean the DC on ADC input from eventually high frequency noise? If yes, how can I connect it for a proper functioning?

Is the first time I use an ADC.. thanks in advance!


1 Answer 1


The ADC parts you are using are recommended for 3V operation not 5V (which I am presuming what you are using, given the 5.1 volt protection zener and attenuation ratio). LT say "For 5V specified devices, see LTC1286 and LTC1298".

Using a zener at 3V is going to be a bit sloppy (due to zener ineffectiveness) so I'd recommend a schottky diode clamp OR choose R2 to be a bit bigger so it can safely attenuate to a voltage within the input range of the ADC.

A low pass filter is probably a good idea - how quickly is the 400V input changing? How fast are you sampling? What are your expectations of performance?

  • \$\begingroup\$ The 400V input is a DC coming from AC/DC trasformers.My samping rate is 7.5ksps (LTC1286). I don't need "super" performances, i only want to eliminate eventually high freqency noise coming from AC/DC (very common). Can you please explain me how can I connect low pass filter? Because is a differential ADC, maybe I need to "clean" both inputs \$\endgroup\$
    – Danny
    Jul 24, 2014 at 10:36
  • \$\begingroup\$ My last phrase make no sense... \$\endgroup\$
    – Danny
    Jul 24, 2014 at 11:12
  • \$\begingroup\$ A capacitor across pins 2 and 3 will do the job. The governing resistance is the 1k, so the low pass filter will start to cut-off when \$\omega = \frac{1}{RC}\$ \$\endgroup\$
    – Andy aka
    Jul 24, 2014 at 11:46

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