ADA4254ACPZ-R7 datasheet(24/59 Pages) AD

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ADA4254

Data Sheet

Rev. B | Page 24 of 59

+IN1

–IN1

+IN2

–IN2

D12

–OUT

PGIA

+OUT

TEST MULTIPLEXER

TEST_MUX[3:0]

AVSS

DVSS

+20mV –20mV

Figure 80. Input Switch Configuration

INPUT MULTIPLEXER

The ADA4254 input multiplexer withstands input voltages up

to ±60 V with respect to VSSH, and 60 V differentially. As shown

in Figure 80, the multiplexer switches between two sets of

inputs and features additional switch functionality on the

output of the multiplexer. Input switching is controlled via the

INPUT_MUX register. The A1, A2, B1, and B2 switches

connect the different inputs to the amplifier. The C1 and C2

switches connect the multiplexer outputs to the test

multiplexer. Switch D12 connects both inputs together. The

input multiplexer features <140 dB of crosstalk.

If excessive input voltage is detected by the input multiplexer,

MUX_OVER_VOLT_ERR in the analog error register trips.

When this error flag is set, the multiplexer automatically opens

A1, A2, B1, and B2 to protect the input amplifier and input

resistor network. This error flag can be disabled by setting

MUX_OVER_VOLT_ERR_DIS. By default, both sets of inputs

cannot be selected simultaneously. This protection can be

overridden via MUX_PROT_DIS.

EMI REDUCTION AND INTERNAL EMI FILTER

In many industrial and data acquisition applications, the ADA4254

amplifies small signals accurately in the presence of large common-

mode voltages or high levels of noise. Typically, the sources of

these very small signals (in the order of microvolts or

millivolts) are sensors that may be a significant distance from

the signal conditioning circuit. Although these sensors may be

connected to signal conditioning circuitry using shielded or

unshielded twisted pair cabling, the cabling may act as an antenna,

conveying very high frequency interference directly to the

inputs of the ADA4254.

The amplitude and frequency of this high frequency interference

can have an adverse effect on the input stage of the instrumentation

amplifier due to unwanted dc shift in the input offset voltage of

the amplifier. This well known effect is called EMI rectification

and is produced when out of band interference is coupled

(inductively, capacitively, or via radiation) and rectified by the

input transistors of the instrumentation amplifier. These transistors

act as high frequency signal detectors, in the same way diodes were

used as RF envelope detectors in early radio designs. Regardless of

the type of interference or the method by which it is coupled to the

circuit, an out of band error signal appears in series with the inputs

of the instrumentation amplifier.

To minimize this effect, the ADA4254 has 35 MHz on-chip

EMI filters to attenuate high frequencies before interacting with

the input transistors. These on-chip filters are well matched due

to their monolithic construction, which minimizes degradation

in ac CMRR. To reduce any further effect of these out of band

signals on the input offset voltage of the ADA4254, an

additional external low-pass filter can be used at the inputs.

Locate the filter very close to the input pins of the circuit. An

effective filter configuration is shown in Figure 81 where three

capacitors are added to the ADA4254 inputs. The filter limits

the input signal according to the following relationship:

Filter FrequencyDIFF =

2(2

)





Filter FrequencyCM =



where:

CD is the differential capacitor and is ≥ 10 CC.

CC is the common-mode capacitor.

CD affects the difference signal. CC affects the common-mode

signal. Any mismatch in R × CC degrades the ADA4254 CMRR.

To avoid inadvertently reducing CMRR bandwidth performance,

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