AD8605(RevD) Просмотр технического описания (PDF) - Analog Devices

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AD8605
(Rev.:RevD)

Precision Low Noise CMOS Rail-to-Rail Input/Output Operational Amplifiers

Analog Devices 

AD8605/AD8606/AD8608

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.

R1

1kΩ

V1

R4

R3

=

R2

R1

R2

VOUT = R1 (V2 – V1)

R2

10kΩ

5V

AD8605

VOUT

R3

1kΩ

V2

R4

10kΩ

Figure 53. Difference Amplifier, AV = 10

120

VSY = ±2.5V

100

AV = 10

80

AV = 1

60

40

20

0

100

1k

10k

100k

1M

10M

FREQUENCY (Hz)

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.

VREF

R

R

R

R2

R2

R2

VOS

CF

RF

V+

AD8605

V–

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 CF 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:

EO

= VOS ⎜⎜⎝⎛1 +

RF

Re q

⎟⎟⎠⎞

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.

15V

RCS

VDD

RFB

OUT1

VREF AD7545

VIN

RP

AGND

DB11

C1

33pF

R3

20kΩ

R2

10kΩ

R1

10kΩ

VOUT

1/2

AD8606

1/2

AD8606

R4

5kΩ10%

Figure 56. Bipolar Operation

Rev. D | Page 17 of 20

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