Introduction
The THS1041 from Texas Instruments (TI) is a 10-bit,
40-MSPS, CMOS high-speed analog-to-digital converter
(ADC). It has many good features, including a single 3-V
supply, low power, a flexible input configuration, a built-in
programmable gain amplifier (PGA), and a built-in clamp
function. Because of these features, especially the built-in
clamp function, the THS1041 has been used in various
applications for many years. The clamp function enables
the device to generate and output a buffered dc voltage
for flexible ADC applications—for example, to provide a
common-mode voltage for the ADC or to allow dc restora-
tion on an ac-coupled video signal at the ADC analog input.
This function can be enabled or disabled. As shown in
Figure 1, the THS1041 clamp function consists of an on-chip
digital-to-analog converter (DAC), logic control, a clamp
input, a buffer, and a clamp output. The clamp output can
be a continuous or interrupted dc signal depending on
whether its Clamp pin receives a dc or pulse signal from an
external source. When this interrupted dc signal is applied
to the ADC single-ended (SE) input circuit to provide
common-mode voltage, the dc stability at the ADC analog
input becomes a concern. Some users have questioned dc
stability when the clamp function and SE input configura-
tion are applied at the same time. This article presents
some test data that explains how the dc voltage behaves in
this kind of application condition and how to get the best
ADC performance when the clamp function is on.