ADA4620-2ARZ-R7 datasheet(43/49 Pages) AD

43 / 49 page

Data Sheet

ADA4620-1, ADA4620-2

analog.com

Rev. 0

43 of 49

trace in Figure 130. The mux-compatible JFET inputs of ADA4620 do not need diode protection, and therefore

remain very high impedance even with the inputs split by several volts and the output slewing, shown in the blue

trace in Figure 130.

Figure 130. Output of High Impedance Buffer During Mux Switching (See the

Figure 128 Circuit)

For a more comprehensive discussion of the benefits of mux-compatible op amps, refer to this KWIK circuit

application note: Mux Compatible Analog Front End.

Third-Order Low-Pass Sallen-Key Filter

Figure 131. Third-Order Butterworth Sallen-Key Filter

A third-order Butterworth filter can be realized without buffering the extra s + 1 term, allowing it to be achieved

with a single op amp, as shown in Figure 131. The noninverting Sallen Key approach is chosen over inverting gain

multiple feedback to preserve the ultrahigh input Z at low frequencies. This third-order approach offers a certain

important advantage over a second-order approach, which omits R1 and C1. The benefit of having R1 and C1 up

front is that it has an extremely wide stop band, limited only by the quality of the components and the layout. As a

result, it keeps extremely high frequencies from getting to R2. If extremely high frequencies are allowed to get to

R2, the only thing to stop them is C2, which can be approximated as a short at high frequencies, and the output

impedance of the op amp. But of course, the output impedance of the op amp is not low enough beyond its

operating frequency range to arrest those frequencies, and the energy in them essentially makes it past the op-amp

output. Figure 132 shows the frequency response of the filter. This design is −3 dB at 100 kHz. Other frequencies can

be selected by scaling the passive values. Of course, it is easiest to scale the resistors because they are readily

available in finer tolerances. The exact calculated value for C3 is 200 pF, but the component value is reduced due to

the contributing effects of trace capacitance and input capacitance.

+15V

–15V

3.01kΩ

1500pF

180pF

2.55kΩ

1.3kΩ

1500pF

ADA4620-1/

ADA4620-2

VOUT

VIN

ADA4620

TRADITIONAL OP AMP

TIME (300ns/DIV)

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