ADA4620-2ARZ-R7 datasheet(40/49 Pages) AD

40 / 49 page

Data Sheet

ADA4620-1, ADA4620-2

analog.com

Rev. 0

40 of 49

Figure 124. Analog and Digital Waveforms Associated with the ADA4620 and LTC2378-20, Showing the Sampling Glitches at 1

MSps

Figure 124 shows the sampling glitches at the IN− of the LTC2378-20 at its full rate of 1 MSps. This is with an exact

and synchronous 5 kHz input waveform, allowing coherence in the time domain (as shown), but this is incoherent

in an FFT (and requires windowing to be intelligible). The red waveform is a piece of the input sinusoid at 5 V/DIV.

The blue waveform is the digital CNV signal clocking the ADC, and the green waveform is the BUSY signal from the

ADC. While BUSY is low, the sampling capacitor is connected to the upstream circuit, and constitutes the analog

acquisition window. The yellow waveform is the actual ADC IN−, with the glitches in evidence. Note that with a 1 µs

sample time, the analog acquisition window is only 300 ns, which is quite a short time for a 16.5 MHz upstream

system to try to settle to 20 bits.

Figure 125. At a Reduced Sampling Rate of 200 kSps, Giving a Large TS of 5 µs, Almost All the Extra Time is Allocated to the

Sampling Window, Allowing Much Better Settling of the Upstream Circuit

Figure 125 shows the same system at a reduced sample rate of 200 kSps. Of the now increased sample time of 5 µs,

the extra 4 µs is allocated almost entirely to the analog acquisition time. This gives ample time for the upstream

16.5 MHz GBW circuit to settle to its most precise values.

VIN (2V/DIV)

CNV (2V/DIV)

BUSY (2V/DIV)

ADC−INPUT (500mV/DIV)

BUSY LOW = ACQUISITION WINDOW