AD8698

Rev. 0 | Page 15 of 20

1k

10k

100k

1M

10M

LOAD CAPACITANCE (nF)

1

10

100

Figure 52. Unity Gain Bandwidth vs. Load Capacitance

Figure 52 shows the unity gain bandwidth as a function of load

capacitance.

INSTRUMENTATION AMPLIFIER

Instrumentation amplifiers are used in applications requiring

precision, accuracy, and high CMRR. One popular application

is signal conditioning in process control, test automation, and

measurement instrumentation, where the amplifier is used to

amplify small signals.

The triple op amp implementation uses the AD8698 at the

front end with the OP184 for optimum accuracy.

The circuit in Figure 53 enjoys a high overall gain, excellent dc

performance, high CMRR, as well as the benefit of an output

that swings to the supplies.

The CMRR of the in-amp will be limited by the choice of

resistor tolerance. R5 is an optional potentiometer that can be

used to calibrate the circuit for maximum gain. R7 can be

trimmed for optimum CMRR.

The output voltage is given by:

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛ +

=

1

R

2

R

4

R

3

R

V

V

IN

O

2

1

V–

V+

V+

R3

9k

Ω

R1

1k

Ω

R2

10k

Ω

R1

9.8k

Ω

R7

400

Ω

R4

2k

Ω

R5

10k

Ω

R3

9k

Ω

V–

V2

V1

V–

V+

OP184

1/2 AD8698

1/2 AD8698

Figure 53. Three Op Amp In-Amp

COMPOSITE AMPLIFIER

The dc accuracy of the AD8698 and the ac performance of the

OP184 are combined in the circuit shown in Figure 54. The

composite amplifier provides a higher bandwidth, a lower offset

voltage, and a higher loop, thereby reducing the gain error

substantially.

The circuit shown exhibits a total output rms noise of less than

500 µV, corresponding to less than 3 mV of peak-to-peak noise

over approximately a 3 MHz bandwidth. Cf is used to minimize

peaking.

The circuit has an inverting gain of 10. In applications with

higher closed-loop gains, Cf is necessary to maintain a

sufficient phase margin and ensure stability. This results in a

narrower closed-loop bandwidth.

V+

OP184

1/2 AD8698

V–

V+

V–

R2

10k

Ω

R1

1k

Ω

Cf

20pF

Figure 54. Composite Amplifier Circuit