REFERENCE CIRCUIT
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.
There are two reference circuits included in the ADS1201
converter: VREF (REFIN, REFOUT) and VBIAS. The circuitry
for VREF 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, VBIAS, is
derived from VREF, whether it is internal or external. VBIAS
is exclusively an output reference. This ratiometric relation-
ship between VREF and VBIAS reduces system errors when
two separate bias voltages are required in the application.
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 REFIN changes with the modulator
clock frequency. The relationship is:
REFERENCE INPUT (REFIN)
The reference input (REFIN) 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-
1E12
Typical REFIN Input Impedance =
50 • fMCLK
fMCLK
X2
X3
X4
X(t)
fS
Integrator 1
Integrator 2
MOUT
VREF
Comparator
X6
D/A Converter
FIGURE 4. Block Diagram of a Second-Order Modulator.
+5V
1
2
3
4
5
6
7
8
AVDD
REFOUT
REFIN
NIC
REFEN 16
MOUT 15
MCLK 14
DVDD 13
DGND 12
CAL 11
1
2
3
4
5
6
7
8
AVDD
REFOUT
REFIN
NIC
REFEN 16
MOUT 15
MCLK 14
DVDD 13
DGND 12
CAL 11
External
VREF
1µF
1µF
ADS1201
ADS1201
AIN
AIN
P
N
AIN
P
N
AIN
AGND
VBIAS
GAIN/OFFSET 10
BIASEN
AGND
VBIAS
GAIN/OFFSET 10
BIASEN
9
9
(a) Internal Reference
(b) External Reference
FIGURE 5. Two Voltage Reference Connection Alternatives for the ADS1201.
®
7
ADS1201
BB [ BURR-BROWN CORPORATION ]
