Analog Input of AD7940.

Question

Hi everyone, 
 I have a query regarding the analog input interface of AD7940. 
 I want to interface of RMS to DC Converter, AD8362 to input of AD7940. Now Input circuit 
 of AD7940 shows a 25 ohm resistor to charge a 25pF capacitor. In that case, the current 
 required to charge the capacitor can go upto 100mA or so. 
 Is it possible to connect the output of AD8362 
 1) Directly to AD7940. 
 OR 
 2) Through some resistance as 100/220 ohm (as for AD8362, RL >= 200 ohm is recommended). 
 OR 
 3) Or some High Current Buffer has to be placed here. 
 Please suggest. Many Regards

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jcolao · Answer

Hi MohitH, 
 The AD7940 is a SAR ADC in which it uses switch capacitors to convert the analog signals. The switching causes some noise on the analog input. We recommend buffer driver on the ADC input to isolate the source from transient voltage during the switching action of sample and hold. We also recommend putting an RC circuit ( e.g. R = 30ohm and C = 56pF) on the analog input to filter high frequency noise. 
 You may also choose to connect directly the AD8362, but I cannot guarantee the performance specially the noise parameter of the system. 
 Kindly refer to the link of an an articl that discuss the front-end of the ADC. 
 http://ez.analog.com/blogs/analogdialogue/2012/12/03/front-end-amplifier-and-rc-filter-design-for-a-precision-sar-ad-converter 
 Regards, 
 Jonathan

enash · Answer

Based on experience with other RMS detectors and ADCs, I think that a direct connection between the detector and the ADC (i.e. no op-amp in between) can work. It would probably be prudent to put pads on your pcb for a series R and a shunt C. I would also suggest that you get the two eval boards and bolt them together. 
 Take a look at Circuit Note CN0178 ( www.analog.com/cn0178 ). It shows the an ADL5902 interfaced to AD7466. In general, I would say that ADL5902 is a better choice than AD8362, mainly because it is more stable vs. temperature and has less transfer function ripple. 
 Here is another thing to think about. the output voltage scaling for both devices is around 50 mV/dB. So a 1 dB change in the input level will change the output by 50 mV. That is a lot of ADC LSBs (AD7940 is 14 bits, so an LSB would be 300 microvolts for 5V fullscale). So even if the SAR does some kick-back, I don't think that it will have much of an impact on overall performance.

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Analog Input of AD7940.