AD8210

Hello,

We are using AD8210 for high side current sensing at   bidirectional dc-dc converter. We have two problems about it. First we   have used 470 pF instead of 400 pF mentioned in the document:   (http://www.analog.com/static/imported-files/application_notes/120739192AN815_0.pdf)   But we saw unexpected high gain. What should be the reason?

Secondly  we saw harmonic on the output signal and we know it is  because of  switching of converter. We use the converter at 20 kHz. Can  we use  input filtering to get rid of these harmonics? If we can use waht   should be the Cfilter? (We have used 10 ohm as Rfilter)

Best regards...

  • Hello. The output capacitance should not be an issue especially since it is a lower capacitive load than what we suggest.

    What is the gain that you are measuring? How much higher than 20V/V?

    Also, what is the switching frequency of the DC/DC? It could be that the common mode voltage switching at the inputs of the AD8210 cause a glitch at the output corresponding to rise/fall edge of input common mode. This could occur as edge of the common mode is so much higher than the bandwidth or the part. This could be helped with some filters at the inputs. Figure 6 of the following attachment shows how such a filter can be implemented.

    http://www.analog.com/library/analogdialogue/archives/42-01/high_side_current_sensing.html

  • I will measure the gain tomorrow. Then I will inform you.

    As I expressed in the question switching frequency of the DC/DC is 20 kHz. I know the External Input Filtering from data sheet but I'm asking how to calculate it?

    Thanks for your help...

  • The issue lies with the rise/fall edge of the 20kHz PWM.

    The frequency of that edge is much higher that the bandwitdth of the AD8210.

    Any filter that has a high enough corner frequency to not interfere with your 20kHz instantenous current will be ok.

    You can try a 1.5MHz 3dB frequency which means your input resistors can stay 10ohm and the capacitor is 0.01uF.Increasing the cap to 0.1uF will lead to 150kHz filter. This would be much better at helping with the rise/fall edge of the PWM, but you will need to check regaring any effects to the instanteous currnt measurement.