AD8605/AD8606/AD8608

Rev. H | Page 19 of 24

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.

AD8605

5V

V2

V1

R1

1kΩ

R3

1kΩ

R2

10kΩ

R4

10kΩ

R4

R3

R2

R1

=

(V2 – V1)

R2

R1

FREQUENCY (Hz)

120

100

0

100

10M

1k

10k

100k

1M

60

40

20

80

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.

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

R2

R2

V+

V–

RR

R

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

for the pole introduced by the output capacitance of the DAC.

adjusted for the best frequency response. The total error at the

output of the op amp can be computed by

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

=

1

Req

R

V

E

F

OS

O

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.

DB11

OUT1

AD7545

AGND

15V

1/2

AD8606

1/2

AD8606

C1

33pF

R4

5kΩ

R2

10kΩ

R1

10kΩ

R3

20kΩ

Figure 56. Bipolar Operation