ADA4255ACPZ-R7 datasheet(26/64 Pages) AD

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Data Sheet

ADA4255

THEORY OF OPERATION

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Rev. 0 | 26 of 64

Figure 86. Input Switch Configuration

INPUT MULTIPLEXER

The ADA4255 input multiplexer withstands input voltages up to

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

Figure 86, the multiplexer switches between the two sets of inputs

and features additional switch functionality on the output of the mul-

tiplexer. 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_ERR 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 (Register ANALOG_ERR_DIS). By

default, both sets of inputs cannot be selected simultaneously. This

protection can be overridden via the MUX_PROT_DIS bit in the

ANALOG_ERR_DIS register.

EMI REDUCTION AND THE INTERNAL EMI

FILTER

In many industrial and data acquisition applications, the ADA4255

amplifies small signals accurately in the presence of large com-

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

of these 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 high

frequency interference directly to the inputs of the ADA4255.

The amplitude and frequency of this high frequency interference

can have an adverse effect on the input stage of the instrumenta-

tion 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 frequen-

cy 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 ADA4255 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 ADA4255, 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 87 where three capacitors are added to the ADA4255

inputs. The filter limits the input signal according to the following

relationship:

Filter FrequencyDIFF= 12πR2CD+CC

(2)

Filter FrequencyCM= 12πRCC

(3)

where:

CD is the differential capacitor and is ≥10CC.

CC is the common-mode capacitor.

CD affects the difference signal, and CC affects the common-mode

signal. Any mismatch in R × CC degrades the ADA4255 CMRR.

To avoid inadvertently reducing CMRR bandwidth performance,

ensure that CC is at least one magnitude smaller than CD. The

effect of mismatched CC values is reduced with a larger CD:CC

ratio.

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