OPA837IDBVT datasheet(40/63 Pages) TI

40 / 63 page

Frequency (Hz)

-51

-45

-39

-33

-27

-21

-15

-9

-3

10k

100k

10M

D083

MFB

SKF

Frequency (Hz)

100

10k

100k

D084

MFB

SKF

OPA837, OPA2837

SBOS673D – SEPTEMBER 2017 – REVISED DECEMBER 2018

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Product Folder Links: OPA837 OPA2837

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Typical Applications (continued)

8.2.1.3 Application Curves

Figure 85 shows the comparative response curves for each of the filter design examples. Both filters hit the

desired response shape exactly. However, notice the loss of stop-band rejection in the Sallen-Key design. This

loss results from the op amp output impedance increasing at higher frequencies and allowing the signal to feed

through the feedback capacitor to the output.

Figure 86 shows a comparison of the output spot noise for the two designs. The Sallen-Key is much lower

because of the lower resistor values used. Also, the MFB shows a noise gain of 2 V/V versus the Sallen-Key

gain of 1 V/V. This difference immediately increases the MFB output noise by at least twice the input voltage

noise from the op amp. The higher resistor values also increase the total output noise for the MFB.

Figure 85. MFB and Sallen-Key Active Filters, Second-

Order, Low-Pass Butterworth Filter Response

Figure 86. Output Spot Noise Comparison

8.2.2 Implementing a 2:1 Active Multiplexer

The OPA837 includes a unique feature that enables a much improved wired-or mux operation. When disabled,

an internal switch opens from the inverting input to the active transistors isolating those nonlinear loads from the

signal being driven back into the inverting input through the active channel. Figure 87 illustrates a simple

example of this multiplexer. In this figure, one of two signals are selected to be passed on to a shared output.

The logic control turns both amplifiers off (logic low) prior to turning one of them on. This control eliminates both

outputs being active at the same time. If both amplifiers must be on, as in the simple switch illustrated in

Figure 87, adding 100-Ω isolating resistors inside the loop at the outputs limits the current flow when both

amplifiers are turned on. This solution offers a very high input impedance to both inputs, very low buffered output

drive, and nearly perfect channel-to-channel isolation. The example of Figure 87 also includes a –0.23-V supply

generator to allow true swing to ground on the output pins. This negative supply generator is optional if the

outputs are more than 0.1 V above ground or intended to be AC-coupled. Testing with a single channel active

and an off channel attached to the output showed no degradation in harmonic distortion; see Figure 17 and

Figure 35. This approach can be expanded to more than two channels or to operate with gain in the channels.

Adding more than two select channels in parallel should add 100-Ω feedback resistors to isolate the inverting

input capacitance from the active output channel.

OPA837IDBVT Datasheet(PDF) 40 Page - Texas Instruments

OPA837IDBVT Datasheet(HTML) 40 Page - Texas Instruments

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