AD8606ARMZ-R7 Analog Devices Inc, AD8606ARMZ-R7 Datasheet - Page 19

• Manufacturer Part Number: AD8606ARMZ-R7
• Description: DUAL, PRECISION CMOS RAIL-RAIL OP AMP
• Manufacturer: Analog Devices Inc
• Series: DigiTrim®r

Specifications of AD8606ARMZ-R7

• Amplifier Type: General Purpose
• Number Of Circuits: 2
• Output Type: Rail-to-Rail
• Slew Rate: 5 V/µs
• Gain Bandwidth Product: 10MHz
• Current - Input Bias: 0.2pA
• Voltage - Input Offset: 80µV
• Current - Supply: 1mA
• Current - Output / Channel: 80mA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• -3db Bandwidth: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

INSTRUMENTATION AMPLIFIERS
The low offset voltage and low noise of the AD8605 make it an
ideal 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. 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.
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.
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.
120
100
80
60
40
20
0
100
R4
R3
V
Figure 54. Difference Amplifier CMRR vs. Frequency
V
OUT
SY
=
V1
V2
R2
R1
= ±2.5V
=
Figure 53. 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
V
= 10
1M
V
OUT
10M
Rev. J | Page 19 of 24
V
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 C
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 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.
V
REF
IN
Figure 55. Simplified Circuit of the DAC8143 with AD8605 Output Buffer
E
R
O
P
=
R2
V
V
R
OS
REF
15V
V
⎛
⎜
⎜
⎝
DD
1
AD7545
R2
+
DB11
F
R
Req
range from 10 pF to 30 pF; it can be adjusted
R
Figure 56. Bipolar Operation
R
F
FB
⎞
⎟
⎟
⎠
R2
AGND
AD8605/AD8606/AD8608
OUT1
R
CS
R
C1
33pF
1/2
AD8606
5kΩ
V
R4
OS
10kΩ
R1
AD8605
R
C
F
F
V+
V–
R3
20kΩ
10kΩ
R2
1/2
AD8606
V
OUT