AD824
A design consideration in sample-and-hold circuits is voltage
droop at the output caused by op amp bias and switch leakage
currents. By choosing a JFET op amp and a low leakage CMOS
switch, this design minimizes droop rate error to better than
0.1 mV/ms in this circuit. Higher values of CH will yield a lower
droop rate. For best performance, CH and C2 should be poly-
styrene, polypropylene or Teflon capacitors. These types of
capacitors exhibit low leakage and low dielectric absorption. Addi-
tionally, 1% metal film resistors were used throughout the design.
less than 2 mV over the entire 0 V to 3.3 V/5 V signal range.
Another method of reducing pedestal error is to reduce the pulse
amplitude applied to the control pins. In order to control the
ADG513, only 2.4 V are required for the “ON” state and
0.8 V for the “OFF” state. If possible, use an input control
signal whose amplitude ranges from 0.8 V to 2.4 V instead of a
full range 0 V to 3.3 V/5 V for minimum pedestal error.
Other circuit features include an acquisition time of less than
3 ms to 1%; reducing CH and C2 will speed up the acquisition
time further, but an increased pedestal error will result. Settling
time is less than 300 ns to 1%, and the sample-mode signal BW
is 80 kHz.
In the sample mode, SW1 and SW4 are closed, and the output
is VOUT = –VIN. The purpose of SW4, which operates in parallel
with SW1, is to reduce the pedestal, or hold step, error by
injecting the same amount of charge into the noninverting input
of A3 that SW1 injects into the inverting input of A3. This
creates a common-mode voltage across the inputs of A3 and is
then rejected by the CMR of A3; otherwise, the charge injection
from SW1 would create a differential voltage step error that
would appear at VOUT. The pedestal error for this circuit is
The ADG513 was chosen for its ability to work with 3 V/5 V
supplies and for having normallyopen and normallyclosed preci-
sion CMOS switches on a dielectrically isolated process. SW2 is
not required in this circuit; however, it was used in parallel with
SW3 to provide a lower RON analog switch.
–14–
REV. C
ADI [ ADI ]
