AD627ARZ-R7 datasheet(19/24 Pages) AD

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AD627

Rev. D | Page 19 of 24

ERRORS DUE TO AC CMRR

In Table 9, the error due to common-mode rejection results

from the common-mode voltage from the bridge 2.5 V. The

ac error due to less than ideal common-mode rejection cannot

be calculated without knowing the size of the ac common-mode

voltage (usually interference from 50 Hz/60 Hz mains frequencies).

A mismatch of 0.1% between the four gain setting resistors

determines the low frequency CMRR of a two-op-amp

instrumentation amplifier. The plot in Figure 43 shows the

practical results of resistor mismatch at ambient temperature.

The CMRR of the circuit in Figure 42 (Gain = +11) was

measured using four resistors with a mismatch of nearly 0.1%

(R1 = 9999.5 Ω, R2 = 999.76 Ω, R3 = 1000.2 Ω, R4 = 9997.7 Ω).

As expected, the CMRR at dc was measured at about 84 dB

(calculated value is 85 dB). However, as frequency increases,

CMRR quickly degrades. For example, a 200 mV p-p harmonic

of the mains frequency at 180 Hz would result in an output

voltage of about 800 μV. To put this in context, a 12-bit data

acquisition system, with an input range of 0 V to 2.5 V, has an

LSB weighting of 610 μV.

By contrast, the AD627 uses precision laser trimming of internal

resistors, along with patented CMR trimming, to yield a higher

dc CMRR and a wider bandwidth over which the CMRR is flat

(see Figure 23).

VOUT

+5V

VIN–

VIN+

–5V

9999.5Ω

999.76Ω

1000.2Ω

9997.7Ω

1/2

OP296

1/2

OP296

Figure 42. 0.1% Resistor Mismatch Example

FREQUENCY (Hz)

120

110

100

10k

100k

Figure 43. CMRR over Frequency of Discrete In-Amp in Figure 42

GROUND RETURNS FOR INPUT BIAS CURRENTS

Input bias currents are dc currents that must flow to bias the

input transistors of an amplifier. They are usually transistor base

currents. When amplifying floating input sources, such as

transformers or ac-coupled sources, there must be a direct dc

path into each input so that the bias current can flow. Figure 44,

Figure 45, and Figure 46 show how to provide a bias current

path for the cases of, respectively, transformer coupling, a

thermocouple application, and capacitive ac-coupling.

In dc-coupled resistive bridge applications, providing this path

is generally not necessary because the bias current simply flows

from the bridge supply through the bridge and into the amplifier.

However, if the impedance that the two inputs see are large, and

differ by a large amount (>10 kΩ), the offset current of the input

stage causes dc errors compatible with the input offset voltage of

the amplifier.

VOUT

TO POWER

SUPPLY

GROUND

–VS

+VS

AD627

REFERENCE

+INPUT

–INPUT

LOAD

Figure 44. Ground Returns for Bias Currents with Transformer Coupled Inputs

VOUT

TO POWER

SUPPLY

GROUND

–VS

+VS

AD627

REFERENCE

+INPUT

–INPUT

LOAD

Figure 45. Ground Returns for Bias Currents with Thermocouple Inputs

VOUT

TO POWER

SUPPLY

GROUND

–VS

+VS

AD627

REFERENCE

+INPUT

–INPUT

100kΩ

LOAD

Figure 46. Ground Returns for Bias Currents with AC-Coupled Inputs

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