AD8605/AD8606/AD8608
Rev. D | Page 17 of 20
INSTRUMENTATION AMPLIFIERS
The low offset voltage and low noise of the AD8605 make it a
great amplifier for instrumentation applications.
Difference amplifiers are widely used in high accuracy circuits
to improve the common-mode rejection ratio.
Figure 53 shows a simple difference amplifier. The CMRR of the
circuit is plotted versus frequency. Figure 54 shows the
common-mode rejection for a unity gain configuration and for
a gain of 10.
Making (R4/R3) = (R2/R1) and choosing 0.01% tolerance yields
a CMRR of 74 dB and minimizes the gain error at the output.
AD8605
5V
V2
V1
R1
1kΩ
R3
1kΩ
R2
10kΩ
R4
10kΩ
V
OUT
R4
R3
R2
R1
=
V
OUT
= (V2–V1)
R2
R1
02731-D-053
Figure 53. Difference Amplifier, A
V
= 10
FREQUENCY (Hz)
120
100
0
100 10M1k
CMRR (dB)
10k 100k 1M
60
40
20
80
A
V
= 10
V
SY
= ±2.5V
A
V
= 1
02731-D-054
Figure 54. Difference Amplifier CMRR vs. Frequency
D/A CONVERSION
The low input bias current and offset voltage of the AD8605
make it an excellent choice for buffering the output of a current
output DAC.
Figure 55 shows a typical implementation of the AD8605 at the
output of a 12-bit DAC.
The DAC8143 output current is converted to a voltage by the
feedback resistor. The equivalent resistance at the output of the
DAC varies with the input code, as does the output capacitance.
R2
AD8605
V
OS
R
F
C
F
R2 R2
V+
V–
02731-D-055
RRR
REF
Figure 55. Simplified Circuit of the DAC8143 with AD8605 Output Buffer
To optimize the performance of the DAC, insert a capacitor in
the feedback loop of the AD8605 to compensate the amplifier
from the pole introduced by the output capacitance of the DAC.
Typical values for C
F
are in the range of 10 pF to 30 pF; it can be
adjusted for the best frequency response. The total error at the
output of the op amp can be computed by the formula:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+=
q
R
VE
F
OSO
Re
1
where Req is the equivalent resistance seen at the output of the
DAC. As mentioned above, Req is code dependant and varies
with the input. A typical value for Req is 15 kΩ. Choosing a
feedback resistor of 10 kΩ yields an error of less than 200 µV.
Figure 56 shows the implementation of a dual-stage buffer at
the output of a DAC. The first stage is used as a buffer.
Capacitor C1, with Req, creates a low-pass filter and thus
provides phase lead to compensate for frequency response. The
second stage of the AD8606 is used to provide voltage gain at
the output of the buffer.
Grounding the positive input terminals in both stages reduces
errors due to the common-mode output voltage. Choosing R1,
R2, and R3 to match within 0.01% yields a CMRR of 74 dB and
maintains minimum gain error in the circuit.
R
FB
V
DD
DB11
OUT1
AD7545
AGND
R
CS
R
P
IN
15V
V
OUT
V
REF
1/2
AD8606
1/2
AD8606
R4
5kΩ10%
R1
10kΩ
R2
10kΩ
R3
20kΩ
C1
33pF
02731-D-056
Figure 56. Bipolar Operation
