REFERENCE CIRCUIT
There are two reference circuits included in the ADS1201
converter: V
REF
(REF
IN
, REF
OUT
) and V
BIAS
. The circuitry
for V
REF
is configured to allow the user to utilize the internal
reference on the chip or provide an external reference to the
converter (see Figure 5). The second reference, V
BIAS
, is
derived from V
REF
, whether it is internal or external. V
BIAS
is exclusively an output reference. This ratiometric relation-
ship between V
REF
and V
BIAS
reduces system errors when
two separate bias voltages are required in the application.
REFERENCE INPUT (REF
IN
)
The reference input (REF
IN
) of the ADS1201 can be config-
ured so that the 2.5V (nominal) internal or external reference
can be used in the conversion process. If the internal refer-
ence is used, the correct connection configuration is shown
in Figure 5a. The capacitor in this circuit is absolutely
required if low noise performance is desired.
An external reference can be used to reduce the noise in the
conversion process. If an external reference is used, care
should be taken to insure that the selected reference has low
noise performance. The appropriate connection circuit of an
external reference is shown in Figure 5b. The reference must
be configured with appropriate capacitors to reduce the high
frequency noise that may be contributed by the reference.
The input impedance of REF
IN
changes with the modulator
clock frequency. The relationship is:
Typical REF
IN
Input Im pedance
=
1E12
50 • f
MCLK
f
MCLK
X
2
X
3
X
4
MOUT
V
REF
Comparator
X(t)
f
S
Integrator 1
Integrator 2
X
6
D/A Converter
FIGURE 4. Block Diagram of a Second-Order Modulator.
+5V
1
2
3
1µF
4
5
6
7
8
NIC
ADS1201
A
IN
P
A
IN
N
AGND
V
BIAS
DGND 12
CAL 11
GAIN/OFFSET 10
BIAS
EN
9
5
6
7
8
A
IN
P
A
IN
N
AGND
V
BIAS
DV
DD
13
AV
DD
REF
OUT
REF
IN
REF
EN
16
MOUT 15
MCLK 14
External
V
REF
1
2
3
1µF
4
NIC
ADS1201
DGND 12
CAL 11
GAIN/OFFSET 10
BIAS
EN
9
DV
DD
13
AV
DD
REF
OUT
REF
IN
REF
EN
16
MOUT 15
MCLK 14
(a) Internal Reference
(b) External Reference
FIGURE 5. Two Voltage Reference Connection Alternatives for the ADS1201.
®
7
ADS1201
BURR-BROWN [ BURR-BROWN CORPORATION ]
