RES31A10DDFR datasheet(29/50 Pages) TI2

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Multiplication of the tERR

effective error by the desired process control value, such as × 6 for a six-sigma approach,

gives conservative maximum bounds. Because the ±1σ values reported in Electrical Characteristics already

include guardbanding and account for mean shifts, in many cases a lower process control value (such as

five-sigma) is sufficient. For example, solving the previous expressions for CMRR yields only 78.3dB, whereas

the actual typical CMRR for the RES31A25 is 89.1dB. The discrepancy arises because the ATE measurement

resolution of tD1, tD2, tM, and CMRR is higher than that of tD2D and tE2E, and therefore the reported typical values

of the latter parameters include additional guardbanding. Additionally, the conservative modeling approach

assumes tD2D, tE2E, and tD2 are uncorrelated, whereas for many devices there are weak correlations (such as

tD2D and tE2E having different polarities) that cause the actual observed error to be lower than the modeled error.

8.1.4 Discrete Instrumentation Amplifiers

The RES31A can be used in conjunction with a dual-channel operational amplifier to implement a discrete

instrumentation amplifier (INA). The ratiometric matching between the two resistor dividers improves CMRR

performance for the circuit when compared to a similar implementation using unmatched discrete resistors,

and results in better overtemperature and overaging gain drift characteristics. INAs are often used instead of

difference amplifiers when high input impedance and low bias currents are needed, such as when measuring

bridge sensors.

Discrete INAs are often configured as a differential-input differential-output circuit as shown in Figure 8-7. While

not shown, if needed, use an additional discrete difference amplifier stage (requiring a second RES31A and

another op-amp channel) to convert the differential output voltage to a single-ended voltage (for example, when

driving a single-ended ADC). This extra stage can also add an additional offset and provide additional gain,

effectively mimicking the common three-amplifier INA architecture.

VOUT+−VOUT−= VIN+−VIN− × 1+ RGRIN

(62)

IN+

ADC

RES31A

–

OPA392

IN–

IN1

IN2

OUT–

–

OPA392

OUT+

Figure 8-7. Differential-Input, Differential-Output Instrumentation Amplifier Using the RES31A

Less commonly, a discrete INA can be implemented as a differential-input, single-ended output circuit as shown

in Figure 8-8. This topology maintains high input impedances, allows an offset to be applied, and gives a

single-ended output without requiring a third amplifier channel. The offset must be driven by a low-impedance

source, such as a reference buffer. When designing a discrete INA, carefully consider the output swing and input

common-mode range limitations of the amplifiers used in the circuit design process.

VOUT= VIN+−VIN− × 1+ RGRIN +VREF

(63)

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