ADA4620-2ARZ-R7 datasheet(42/49 Pages) AD

42 / 49 page

Data Sheet

ADA4620-1, ADA4620-2

analog.com

Rev. 0

42 of 49

Each time the multiplexer switches channels, the multiplexed signal changes value. Even if the input signals are

stable, the multiplexed signal varies, requiring downstream circuitry to respond quickly to these transitions. If the

output signal does not settle to the target accuracy before reading the next channel, the measured value of a

channel can be affected by the previous channel's value, causing channel-to-channel crosstalk. To prevent this, an

op amp used as a high impedance stage at the multiplexer’s output must respond quickly to its output. The

ADA4620 op amp is highly suitable for this purpose due to its low propagation delay and high slew rate,

maintaining measurement precision. Although high-speed op amps usually consume a lot of power, the ADA4620

provides a high slew rate while powered with a low supply current, enabling high throughput in multiplexed DAQ

systems with lower power consumption. The proprietary circuit topology of this amplifier gives excellent slew rate

at low quiescent power dissipation without compromising precision or settling time.

Figure 128. Multiplexed System and Different Input Structures of Op Amp

Even if the op amp following the multiplexer is fast enough, another crucial detail often overlooked is the input

structure of the operational amplifier. The high impedance stage op amp can be exposed to large differential

voltages due to the multiplexer switching between significantly different DC voltages. Therefore, the high

impedance stage op amp should be capable to tolerate and measure large step input voltages. The ADA4620

addresses this issue with a robust, mux-compatible architecture that can handle large differential voltages up to

the supply rails without relying on differential back-to-back diodes. See Figure 128.

Figure 129. Traditional Op Amp Inrush Current vs. Small ADA4620 Mux-Compatible Glitch Representing Mux Switching Event

In a traditional op amp, this large but temporary differential voltage between the input pins results in inrush

current due to diode conduction while the op amp output is slewing, as seen in the red trace in Figure 129. This

effect causes charge errors to form on upstream RCs, which takes time to settle. This effect can be seen in the red

ANTI-PARALLEL

DIODES

TRADITIONAL

OPERATIONAL

AMPLIFIER

ADC

RMUX

RSOURCE

+5V

–5V

ADA4620-1/

ADA4620-2

ADC

RMUX

RSOURCE

+5V

–5V

ADA4620

TRADITIONAL OP AMP

TIME (400ns/DIV)

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