2
The sample-hold was configured as a unity gain follower
with its input grounded. The output trace displays a pedestal
error of –6mV. Note that this is a negative excursion due to
the rising edge of the digital mode control signal coupling to
the inverting input of the sample-hold circuit.
The best way to avoid this is to solder the sample-hold
directly into the board. If hard soldering is not a viable
option, avoid the use of high profile sockets, especially the
zero insertion force type. The optimum approach uses zero
profile solderless sockets (such as Augut P/N 8134-HC-
5P2).
The waveforms in Figure 4 confirm that a test board built
with the zero profile pin connectors shows no evidence of
signal coupling from the digital control pin to the inverting
input. The board layout uses the technique mentioned earlier
of routing a minimum width feedback trace underneath the
circuit. In this example, the socket is empty and the signals
are measured just as they were in Figure 2.
Figure 5 demonstrates the reward for the attention to detail
covered by this discussion. The pedestal is a barely percep-
tible –0.5mV for the same unity gain configuration and for
the same unit as illustrated in Figure 3.
FIGURE 4. Signal Coupling from Pin 14 to Pin 1 for Empty
Zero Profile Socket.
FIGURE 5. Pedestal Error Resulting from the Use of a Zero
Profile Socket.
–Input
5mV/div
0V
S/H Input
2V/div
0V
100
μ
s/div
100
μ
s/div
V
5mV/div
0V
S/H Input
2V/div
0V
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no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.