By Bonnie C. Baker
difficult to reject in a single supply circuit. A DC pedestal
tional amplifier with a small signal riding on top of it. This
occur. In order to digitize this type of signal, the DC offset
as 12 bits. The
鈭單?/div>
converters, ADS1210, ADS1211,
ADS1212, or ADS1213, are appropriate for this function.
An alternative to using a high resolution converter would be
to add an analog stage where the signal is level shifted to
eliminate the DC offset. Unfortunately, this may complicate
matters further in creating a situation where dual supplies
are required.
+5V
A better solution to rejecting the DC pedestal voltage while
still digitizing the small signal is to use one device and a few
passive elements. The circuit configuration, shown in Figure
1, shows how to use a differential input, 12-bit, A/D con-
verter, two resistors and a capacitor to accomplish this final
goal. In Figure 1, the single-ended output of a standard
single-supply operation amplifier, such as an OPA234, is
connected to the inputs of a differential-input A/D converter
through an R || C network. The R || C network in conjunction
with the inputs of the A/D converter is designed to reject
lower frequency portions of the signal while still digitizing
the signal鈥檚 higher frequencies.
In Figure 1, a single supply amplifier, A
1
, is used as a buffer
in the signal path. The output signal of the amplifier is split
in two. The first signal path is through a resistor (R
2
) to the
non-inverting input of the differential input, 12-bit A/D
converter (pin 2 of the ADS7817). The second signal path is
3
4
1k鈩?/div>
0.1碌F
5碌F
REF1004-1.2
DC and AC
Signal
R
2
2
3
A1
6
R
1
OPA234
C
1
0.1碌F
Low Pass
Single Pole
Filter
3
鈥揑N
GND
4
DC + Low
Frequency
CS/SHDN
1
V
REF
8
8
+V
CC
DOUT
DCLOCK
7
5
WST
WSR
2
+IN
SDI 0
SCKT
SDO 0
ADS7817
V
IN
SD02
SS/HA2
Motorolla
DSP56004
FIGURE 1. The ADS7817 in Conjunction with an R || C Network is Used to Reject DC Pedestal Voltages in a Single Supply
Environment and Digitize the ac Portion of the Analog Signal. The REF1004-1.2 is a 1.2 voltage reference for the
A/D converter, providing a full-scale range of 2.4Vp-p and theoretical LSB size of 0.6mV.
漏
1997 Burr-Brown Corporation
AB-118
Printed in U.S.A. June, 1997
1
2
