Datasheet AD8610, AD8620 (Analog Devices) - 20

link to page 20 link to page 20 link to page 20
AD8610/AD8620 High Speed Instrumentation Amplifier
In active filter applications using operational amplifiers, the dc The 3-op-amp instrumentation amplifiers shown in Figure 68 can accuracy of the amplifier is critical to optimal filter performance. provide a range of gains from unity up to 1000 or higher. The The offset voltage and bias current of the amplifier contribute to instrumentation amplifier configuration features high common- output error. Input offset voltage is passed by the filter and can mode rejection, balanced differential inputs, and stable, accurately be amplified to produce excessive output offset. For low frequency defined gain. Low input bias currents and fast settling are achieved applications requiring large value input resistors, bias and offset with the JFET input AD8610/AD8620. Most instrumentation currents flowing through these resistors also generate an offset amplifiers cannot match the high frequency performance of this voltage. circuit. The circuit bandwidth is 25 MHz at a gain of 1, and close to At higher frequencies, the dynamic response of the amplifier 5 MHz at a gain of 10. Settling time for the entire circuit is 550 ns to must be carefully considered. In this case, slew rate, bandwidth, 0.01% for a 10 V step (gain = 10). Note that the resistors around and open-loop gain play a major role in amplifier selection. the input pins need to be small enough in value so that the RC The slew rate must be both fast and symmetrical to minimize time constant they form in combination with stray circuit capaci- distortion. The bandwidth of the amplifier, in conjunction with the tance does not reduce circuit bandwidth. gain of the filter, dictates the frequency response of the filter. The
V+
use of high performance amplifiers, such as the AD8610/AD8620, minimizes both dc and ac errors in all active filter applications.
8 +INA 3 Second-Order, Low-Pass Filter 1/2 AD8620 1 U1
Figure 69 shows the AD8610 configured as a second-order,
2 4
Butterworth, low-pass filter. With the values as shown, the
C5 10pF
design corner was 1 MHz, and the bench measurement was
V– V+
974 kHz. The wide bandwidth of the AD8610/AD8620 allows
R1
corner frequencies into the megahertz range, but the input
1kΩ 7
capacitances should be taken into account by making C1 and
3 R4 V R7 C4 OUT
C2 smaller than the calculated values. The following equations
2kΩ AD8610 2kΩ 15pF 6 U2
can be used for component selection:
2 R6 RG 4 2kΩ R8
R1 = R2 = User Selected (Typical Values = 10 kΩ to 100 kΩ)
2kΩ V–
1.414
R5
C1 =
2kΩ
( π 2 )( f )( ) R1 CUTOFF
+INB 5 C3
0.707
1/2 AD8620 15pF
=
7
C2
U1
( π 2 )( f )( ) R1 CUTOFF
6 R2
where C1 and C2 are in farads.
1kΩ +13V C1
68 0
220pF C2
0- 73
10pF R2 R1 7
02
1020Ω 1020Ω
Figure 68. High Speed Instrumentation Amplifier
VIN 3 5 C2 110pF AD8610 6 V High Speed Filters OUT U1 2 1
The four most popular configurations are Butterworth, Elliptical,
4
9 Bessel (Thompson), and Chebyshev. Each type has a response -06 30
–13V
that is optimized for a given characteristic, as shown in Table 6. 027 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 Rev. F | Page 20 of 24 Document Outline FEATURES APPLICATIONS PIN CONFIGURATIONS GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION FUNCTIONAL DESCRIPTION Power Consumption Driving Large Capacitive Loads Slew Rate (Unity Gain Inverting vs. Noninverting) Input Overvoltage Protection No Phase Reversal THD Readings vs. Common-Mode Voltage Noise vs. Common-Mode Voltage Settling Time Output Current Capability Operating with Supplies Greater than ±13 V Input Offset Voltage Adjustment Programmable Gain Amplifier (PGA) High Speed Instrumentation Amplifier High Speed Filters Second-Order, Low-Pass Filter High Speed, Low Noise Differential Driver OUTLINE DIMENSIONS ORDERING GUIDE