ADS62P49 footprin

Hi,

For the center tap on primary as well as secondary windings, the EVM has the pad and connection just for flexibility to choose other transformers or baluns later that may be more suited for a particular application.  Most of the transformers we use will have a center tap only on the secondary, and the pad for the possible center tap on the primary has no connection on the transformer so there is no harm in the extra pad.  But there are a few transformers i have seen that have a center tap on both primary and secondary.  Our transformer footprint is just designed for flexibility.

I think the center tap has the bypass cap to help filter out common mode noise.

The optimal values of the RCR filter to filter out sampling noise is dependent on the sampling rate and input frequencies of interest.  The RCR filter is somewhat a low pass filter, so the values are to be chosen that the bandwidth of the RCR is flat for the desired input frequencies, and then to roll off above that.   We don't want the RCR circuit to attenuate the desired input frequencies, so we set the bandwidth accordingly (normally by leaving the R=50ohms and choosing the appropriate value of C) to pass the input and filter out above that, regardless of the sample rate.  The sampling glitches are caused by the rising and falling edges of the sample clock, so we do know a lot about what frequencies the sampling glitches wil have (harmonics of the clock) but still we have to let the RCR pass the desired input bandwidth and we don't need to pass anything above the input frequencies so we set the RCR according to input frequency.

By definition, the *peak to peak* differential voltage is 2V if each side of the differential signal swings 1V each.  So if at any time you define the input voltage as the 'P' side minus the 'M' side, then this voltage can befrom +1.0V to -1.0V, so the peak to peak difference is 2V.  And if each side of the differential signal has a 1V swing around a common mode, then the swing is from 0.5V below common mode to 0.5V above common mode.

In that schematic, R5, R6 and C12 is just another RCR filter, but only one of the RCR filters is to be soldered in at any one time.  If you look at the datasheet for the ADS62P49 in the applications section, there is a typical input circuit for high input frequency (figure97) and another circuit for low frequency (figure96) input.  One difference is that the RCR filter is before the series 15Ohm in one case and after the series 5 ohm in the other case.  We built the EVM to be able to support either circuit.  

Regards,

Richard P.

Dennis,

for the ADS62P49 you actually need both RCR filters.

The first one (R5R6C12) is the 50ohm termination of the input transmission line. This also has the AC termination to the common mode voltage (VCM) of the ADS62P49.

The second RCR filter (R25R26C65) is designed to take care of the charge kickback of the ADC internal front end itself. If you compare several of our CMOS ADCs you'll notice that this RCR filter will vary from device to device and it also depending on input frequency itself.

Please let us know if you have any further questions.

Thomas Neu