Question from User:
From my understanding of the AD630 and ADA2200 datasheets, the highest carrier frequency of ADA2200 is 32kHz while the AD630 is 2 MHz. Why is the ADA2200 recommended for higher frequency applications? Is my understanding of the datasheets wrong?
The General Description section of the AD630 datasheet states, "Although optimized for operation up to 1 kHz, the circuit is useful at frequencies up to several hundred kilohertz." So the circuit will function at higher frequencies than the default ADA2200 operation. The AD630 will not work well as a synchronous demodulator up to 2MHz. The 2MHz figure is the unity gain bandwidth of the output amplifier.
In its normal operational mode, the ADA2200 demodulates the input signal at the REFCLK frequency. The REFCLK signal is limited to 32KHz, which would limit the input carrier signal (excitation frequency) to 32KHz as well.
However, the ADA2200 can be used to demodulate signals with higher excitation frequencies by using the ADA2200 in an undersampling mode. By this I mean, run the ADA2200 at an input clock rate exactly equal to the excitation frequency. Then the input sampling of the ADA2200 would immediately downconvert the signal measurement to dc. The decimation filter would lower the output sample rate to fclkin/8. The tunable filter could be used to lower the bandwidth of the signal. You could use an analog output filter for additional noise limiting. Because you are undersampling, the amount of high frequency noise rejection from an AAF would be limited compared to the normal operating mode, but you could get good low frequency rejection with an input HPF.
Input signal carrier frequencies up to 1MHz can be demodulated in this way since the input clock rate of the ADA2200 supports a minimum of 1MHz.