Datasheet

AD8295
Rev. A | Page 23 of
APPLICATIONS INFORMATION
CREATING A REFERENCE VOLTAGE AT MIDSCALE HIGH ACCURACY G = −1 CONFIGURATION WITH
LOW-PASS FILTER
A reference voltage other than ground is often useful, for
example, when driving a single-supply ADC. Creating a
reference voltage derived from a voltage divider is straight-
forward with the AD8295 (see Figure 67). In this configuration,
Op Amp A2 is used to provide a buffered V
S
/2 reference for the
in-amp section. This configuration is very similar to the one
described in the Reference Terminal section.
The circuit in Figure 68 uses Op Amp A1 and the resistor string
to provide a precise G = −1 configuration. Because no external
resistors are used to set the gain, gain accuracy and gain drift
depend only on the internally matched resistors, yielding excel-
lent performance.
Adding a capacitor across Resistor R2 is a simple way to provide
a single-pole low-pass filter that rolls off at 20 dB per decade.
This capacitor is shown as C1 in Figure 68.
Note that the internal resistors of Op Amp A1 are not used
to provide V
S
/2. Instead, external 1% (or better) resistors are
used. Because the negative input of Op Amp A1 is permanently
connected to the junction of internal resistors R1 and R2,
Op Amp A1 operates as a low voltage clamp, preventing the
resistor string from providing a convenient V
S
/2 voltage.
A1
OUT
A1
R2
A2
+IN
A2
–IN
–IN
28
Noise at the reference feeds directly to the output, so if the
reference voltage is derived from a noisy source, filtering is
required. In Figure 67, Capacitor C1 has been added to filter
out high frequency noise on the positive power supply line.
The 10 F capacitor and the 100 k resistors shown in Figure 67
roll off noise starting at 0.3 Hz. The filter frequency is a trade-
off between noise rejection and start-up time.
A1 A1
A2
+IN
A2
–IN
+V
S
+
OUT R2
V
S
OUT
REF
1
2
3
4
865
131416
A1
IA
A2
R1
20kΩ
R2
20kΩ
100kΩ
100kΩ
A2
OUT
C1
10µF
AD8295
A1
+IN
A1
R1
A1
–IN
12
11
10
9
0907343-0
7
–IN
–INPUT
OUTPUT
+INPUT
R
G
R
G
+IN
15
+V
S
0.1µF
V
S
/2
BUFFERED
Figure 67. Single-Supply Connection with Buffered Reference
–INPUT
+INPUT
R
G
+V
S
–V
S
OUT
R
G
+IN
16 15 14 13
1
2
3
4
8765
A1
IA
A2
R1
20kΩ
R2
20kΩ
A2
OUT
AD8295
V
REF
INPUT
V
REF
INPUT
LOW-PASS
FILTERED
OUTPUT
V
REF
BUFFERED
C1
A1
R1
A1
–IN
12
11
10
9
NOTES
1.
f
LOW-PASS
= 1/(2π 20kΩ C1).
07343-011
Figure 68. Single-Pole Output Filter Using a Single External Capacitor
If the connections to Pin 10 and Pin 11 in Figure 68 are changed
so that Pin 10 connects to ground and Pin 11 connects to the
in-amp output, the result is a G = 2 circuit, also with excellent
gain accuracy and drift. In the G = 2 configuration, Capacitor C1
lowers the gain from 2 to 1 at higher frequencies.