AD8605/AD8606/AD8608
INSTRUMENTATION AMPLIFIERS
The low offset voltage and low noise of the AD8605 make it an
ideal amplifier for instrumentation applications.
R
R
R
V
REF
C
F
R
F
Difference amplifiers are widely used in high accuracy circuits
to improve the common-mode rejection ratio. Figure 53 shows
a simple difference amplifier. Figure 54 shows the common-
mode rejection for a unity gain configuration and for a gain of 10.
R2
R2
R2
V+
V
OS
AD8605
Making (R4/R3) = (R2/R1) and choosing 0.01% tolerance yields
a CMRR of 74 dB and minimizes the gain error at the output.
R1
R2
1kΩ
10kΩ
V–
V1
Figure 55. Simplified Circuit of the DAC8143 with AD8605 Output Buffer
5V
R4 R2
=
To optimize the performance of the DAC, insert a capacitor in
the feedback loop of the AD8605 to compensate the amplifier
for the pole introduced by the output capacitance of the DAC.
Typical values for CF range from 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
R3 R1
R2
R1
AD8605
V
OUT
V
=
(V2 – V1)
OUT
R3
1kΩ
R4
10kΩ
V2
⎛
⎜
⎝
⎞
⎟
⎟
⎠
RF
Req
⎜
EO =VOS 1 +
Figure 53. Difference Amplifier, AV = 10
120
100
80
V
= ±2.5V
SY
where Req is the equivalent resistance seen at the output of the
DAC. As previously mentioned, Req is code dependent and
varies with the input. A typical value for Req is 15 kꢀ.
Choosing a feedback resistor of 10 kꢀ yields an error of less
than 200 μV.
A
= 10
V
A
= 1
V
60
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.
40
20
0
100
1k
10k
100k
1M
10M
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.
FREQUENCY (Hz)
Figure 54. Difference Amplifier CMRR vs. Frequency
DAC 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.
R
R3
20kΩ
CS
15V
Figure 55 shows a typical implementation of the AD8605 at the
output of a 12-bit DAC.
R2
10kΩ
C1
33pF
V
R
FB
DD
R1
OUT1
AGND
10kΩ
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.
V
AD7545
OUT
V
REF
R
V
P
IN
1/2
DB11
1/2
AD8606
AD8606
R4
5kΩ
Figure 56. Bipolar Operation
Rev. H | Page 19 of 24
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