AMC1200: AMC1200 input anti-aliasing RC filter

Yang Wu

Part Number: AMC1200

Hi Team,

My customer is using AMC1200 to monitor the voltage of battery pack (400V). About the input anti-aliasing RC filter, we found some interesting things as bellow:

1) Figure A, with RC filter (10ohm, 330pF), the sampling result is accurate, ±2V variance.

2) Figure A, without RC filter (10ohm, 330pF), the sampling result is accurate, ±2V variance.

3) Figure B, short 2 Resistor, with only C filter (330pF), the sampling result has -20V to -5V bias, bellow is the sampling results.

Could you help comment why the result will have bias if only has C filter (I have run the simulation and found that the frequency response of case 2) & 3) are the same)?

		Result	VOUTP	VOUTN	VOUTP - VOUTN
Test 1	AMC1200 Sampling	338V	3.162V	2.039V	1.123V
Test 1	Actual Value	350V	3.13V	1.97V	1.16V

Test 2	AMC1200 Sampling	188V	2.91V	2.288V	0.622V
Test 2	Actual Value	200V	2.8814V	2.2186V	0.6628V

Thanks a lot.

Best regards,

Yang

over 2 years ago

0 Alexander Smith over 2 years ago

TI__Mastermind 48791 points

Hi Yang,

The input of the AMC1200 is a switched capacitor, somewhat similar to a SAR converter. As stated in the datasheet, the external RC filter adds stability and reduces charge dumping to the inputs and the charging of C_IND. Without these resistors, the charging of C_IND may be unstable due to the large capacitor C5 and take a longer time to settle than the AMC1200 samples. Section 8 of the precision lab series goes into further detail: https://training.ti.com/ti-precision-labs-adcs-introduction-sar-adc-front-end-component-selection?context=1139747-1140267-1128375-1139106-1128643

0 Yang Wu over 2 years ago in reply to Alexander Smith

TI__Intellectual 2960 points

Hi Alex,

thanks for your help. However, can customer remove both resistor & capacitor ( case 2) Figure A, without RC filter (10ohm, 330pF), the sampling result is accurate, ±2V variance) ?

Thanks a lot.

Best regards,

Yang

0 kai klaas69 over 2 years ago in reply to Yang Wu

Guru 140200 points

Hi Yang,

the RC filter not only works as a charge bucket filter for the input voltage sampling switchings but also isolates the rest of the circuitry from the switching noise. So it's not at all wise to omit the input filter.

The TI's designers have spent lots of time and energy to make the AMC1200 work as properly as possible. So why not just add these simple three components the datasheet recommends?

Kai

0 Yang Wu over 2 years ago in reply to kai klaas69

TI__Intellectual 2960 points

Hi Kai,

thanks for your help! Because customer has already done for PCB manufacturing without RC filters. I will ask customer to redesign it.

Best regards,

Yang

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AMC1200: AMC1200 input anti-aliasing RC filter