REV. D

AD8610/AD8620

–15–

High Speed Instrumentation Amplifier (IN AMP)

The three op amp instrumentation amplifiers shown in Figure 28

can provide a range of gains from unity up to 1,000 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

capacitance does not reduce circuit bandwidth.

1/2 AD8620

C2

10pF

R2 1k

R4 2k

R7

2k

C4

15pF

R6

2k

R8 2k

V–

V+

AD8610

U2

C3

15pF

R5 2k

V–

V+

1/2 AD8620

U1

C5

10pF

R1 1k

RG

Figure 28. High Speed Instrumentation Amplifier

High Speed Filters

The four most popular configurations are Butterworth, Elliptical,

Bessel, and Chebyshev. Each type has a response that is optimized

for a given characteristic as shown in Table II.

In active filter applications using operational amplifiers, the

dc accuracy of the amplifier is critical to optimal filter performance.

The amplifier’s offset voltage and bias current contribute to output

error. Input offset voltage is passed by the filter, and may be

amplified to produce excessive output offset. For low frequency

applications requiring large value input resistors, bias and offset

currents flowing through these resistors will also generate an

offset voltage.

At higher frequencies, an amplifier’s dynamic response 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

amplifier’s bandwidth, in conjunction with the filter’s gain, will

dictate the frequency response of the filter. The use of a high perfor-

mance amplifier such as the AD8610/AD8620 will minimize both

dc and ac errors in all active filter applications.

Second-Order Low-Pass Filter

Figure 29 shows the AD8610 configured as a second-order

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

frequency of the filter will be 1 MHz. The wide bandwidth of

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

megaHertz. The following equations can be used for component

selection:

R1

R2

C1

1

C2

1

==

−

()

)( )

)( )

User Selected Typical Values

fR

fR

CUTOFF

CUTOFF

:k

k

.

.

10

100

1 414

2

0 707

2

ΩΩ

π

π

where C1 and C2 are in farads.

–13V

+13V

5

C2

11pF

AD8610

U1

R2

10k

R1

10k

C1

22pF

Figure 29. Second-Order Low-Pass Filter

Table II. Filter Types

Type

Sensitivity

Overshoot

Phase

Amplitude (Pass Band)

Butterworth

Moderate

Good

Max Flat

Chebyshev

Good

Moderate

Nonlinear

Equal Ripple

Elliptical

Best

Poor

Equal Ripple

Bessel (Thompson)

Poor

Best

Linear