AD8608 Analog Devices, AD8608 Datasheet - Page 20

• Manufacturer Part Number: AD8608
• Description: Precision, Low Noise, CMOS, Rail-to-Rail, Input/Output Operational Amplifier (Quad)
• Manufacturer: Analog Devices

Specifications of AD8608

• Vcc-vee: 2.7V to 6V
• Isy Per Amplifier: 1.2mA
• Packages: SOIC,SOP
• -3db Bandwidth: 10MHz
• Slew Rate: 5V/µs
• Vos: 20µV
• Ib: 200fA
• # Opamps Per Pkg: 4
• Input Noise (nv/rthz): 6.5nV/rtHz

AD8605/AD8606/AD8608
INSTRUMENTATION AMPLIFIERS
The low offset voltage and low noise of the
ideal amplifier for instrumentation applications.
Difference amplifiers are widely used in high accuracy circuits
to improve the common-mode rejection ratio. Figure 54 shows
a simple difference amplifier. Figure 55 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.
DAC CONVERSION
The low input bias current and offset voltage of the
make it an excellent choice for buffering the output of a current
output DAC.
Figure 56 shows a typical implementation of 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.
120
100
80
60
40
20
0
100
R4
R3
V
Figure 55. Difference Amplifier CMRR vs. Frequency
V
OUT
SY
=
V1
V2
R2
R1
= ±2.5V
=
Figure 54. Difference Amplifier, A
R2
R1
1k
(V2 – V1)
1kΩ
1kΩ
R1
R3
10k
FREQUENCY (Hz)
A
A
V
AD8605
V
= 10
= 1
5V
100k
10kΩ
10kΩ
R2
R4
AD8605
V
= 10
1M
AD8605
V
OUT
AD8605
make it an
10M
at the
Rev. L | Page 20 of 24
V
To optimize the performance of the DAC, insert a capacitor in
the feedback loop of the
for the pole introduced by the output capacitance of the DAC.
Typical values for C
adjusted for the best frequency response. The total error at the
output of the op amp can be computed by
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 kΩ.
Choosing a feedback resistor of 10 kΩ yields an error of less
than 200 µV.
Figure 57 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
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.
V
IN
REF
Figure 56. Simplified Circuit of the DAC8143 with
E
R
O
P
=
R2
V
V
R
OS
REF
15V
V




DD
1
AD7545
R2
+
DB11
R
Req
F
Figure 57. Bipolar Operation
R
range from 10 pF to 30 pF; it can be
R
F
FB
AD8606




R2
AD8605
AGND
OUT1
R
R
CS
is used to provide voltage
to compensate the amplifier
C1
33pF
1/2
AD8606
5kΩ
V
R4
OS
10kΩ
AD8605
R1
AD8605
Data Sheet
R
C
F
F
V+
V–
R3
20kΩ
Output Buffer
10kΩ
1/2
AD8606
R2
V
OUT