AD8610/AD8620

High Speed Instrumentation Amplifier

The three op amp instrumentation amplifiers shown in Figure 68

can provide a range of gains from unity up to 1000 or higher. The

instrumentation amplifier configuration features high common-

mode rejection, balanced differential inputs, and stable, accurately

defined gain. Low input bias currents and fast settling are achieved

with the JFET input AD8610/AD8620. Most instrumentation

amplifiers cannot match the high frequency performance of this

circuit. The circuit bandwidth is 25 MHz at a gain of 1, and close to

5 MHz at a gain of 10. Settling time for the entire circuit is 550 ns to

0.01% for a 10 V step (gain = 10). Note that the resistors around

the input pins need to be small enough in value so that the RC

time constant they form in combination with stray circuit capaci­

tance does not reduce circuit bandwidth.

V+

In active filter applications using operational amplifiers, the dc

accuracy of the amplifier is critical to optimal filter performance.

The offset voltage and bias current of the amplifier contribute to

out-put error. Input offset voltage is passed by the filter, and can

be amplified to produce excessive output offset. For low frequency

applications requiring large value input resistors, bias and offset

currents flowing through these resistors also generate an offset

voltage.

At higher frequencies, the dynamic response of the amplifier

must be carefully considered. In this case, slew rate, bandwidth,

and open-loop gain play a major role in amplifier selection. The

slew rate must be both fast and symmetrical to minimize

distortion. The bandwidth of the amplifier, in conjunction with the

gain of the filter, dictates the frequency response of the filter. The

use of high performance amplifiers such as the AD8610/AD8620

minimizes both dc and ac errors in all active filter applications.

1/2 AD8620

R2

1kΩ

R4

2kΩ

C4

15pF

R8

2kΩ

R7

2kΩ

R1

1kΩ

C5

10pF

V–

V+

AD8610

U2

C3

15pF

R5

2kΩ

R6

2kΩ

V–

1/2 AD8620

U1

RG

5

6

7

U1

7

4

6

3

2

8

4

1

3

2

Second-Order Low-Pass Filter

Figure 69 shows the AD8610 configured as a second-order,

Butterworth, low-pass filter. With the values as shown, the corner

frequency of the filter is 1 MHz. The wide bandwidth of the

AD8610/AD8620 allows a corner frequency up to tens of mega­

hertz. The following equations can be used for component

selection:

R1 = R2 = User Selected

1.414

2π

0.707

2π

R1

where C1 and C2 are in farads.

AD8610

7

4

6

1

5

2

3

+13V

–13V

C1

22pF

R1

10kΩ

U1

R2

10kΩ

C2

10pF

C2

Figure 68. High Speed Instrumentation Amplifier

11pF

High Speed Filters

The four most popular configurations are Butterworth, Elliptical,

Bessel (Thompson), and Chebyshev. Each type has a response

Figure 69. Second-Order Low-Pass Filter

that is optimized for a given characteristic as shown in Table 6.

Table 6. Filter Types

Type

Sensitivity

Overshoot

Phase

Amplitude (Pass Band)

Butterworth

Chebyshev

Elliptical

Bessel (Thompson)

Moderate

Good

Best

Poor

Good

Moderate

Poor

Best

Nonlinear

Linear

Max Flat

Equal Ripple

Equal Ripple

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