AD8610/AD8620
Rev. F | Page 20 of 24
High Speed Instrumentation Amplifier
The 3-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.
02730-068
1/2 AD8620
+INB
R2
1kΩ
C2
10pF
R4
2kΩ
C4
15pF
V
OUT
R8
2kΩ
R7
2kΩ
R1
1kΩ
C5
10pF
V–
V+
AD8610
U2
C3
15pF
R5
2kΩ
R6
2kΩ
+INA
V–
+
1/2 AD8620
U1
RG
5
6
7
U1
7
4
6
3
2
8
4
1
3
2
Figure 68. High Speed Instrumentation Amplifier
High Speed Filters
The four most popular configurations are Butterworth, Elliptical,
Bessel (Thompson), and Chebyshev. Each type has a response
that is optimized for a given characteristic, as shown in Table 6.
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
output 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.
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
design corner was 1 MHz, and the bench measurement was
974 kHz. The wide bandwidth of the AD8610/AD8620 allows
corner frequencies into the megahertz range, but the input
capacitances should be taken into account by making C1 and
C2 smaller than the calculated values. The following equations
can be used for component selection:
R1 = R2 = User Selected (Typical Values = 10 k to 100 k)
()
()
()
R1f
C1
CUTOFF
π
2
414.1
=
()
()
()
R1f
C2
CUTOFF
π
2
707.0
=
where C1 and C2 are in farads.
V
IN
V
OUT
AD8610
7
4
6
1
5
2
3
+13V
–13V
C2
110pF
C1
220pF
02730-069
R2
1020Ω
R1
1020Ω
U1
Figure 69. Second-Order, Low-Pass Filter
Table 6. Filter Types
Type Sensitivity Overshoot Phase Amplitude (Pass Band)
Butterworth Moderate Good Maximum flat
Chebyshev Good Moderate Nonlinear Equal ripple
Elliptical Best Poor Equal ripple
Bessel (Thompson) Poor Best Linear
