INA821_19 datasheet(21/43 Pages) TI1

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Frequency (Hz)

100

120

140

10M

100M

10G

Frequency (Hz)

100

120

10M

100M

10G

EMIRR (dB)

RF _ PEAK

100 mV

INA821

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SBOS893B – AUGUST 2018 – REVISED MAY 2019

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8.3.1.1 Gain Drift

The stability and temperature drift of the external gain setting resistor (RG) also affects gain. The contribution of

RG to gain accuracy and drift is determined from Equation 1.

The best gain drift of 5 ppm/

℃ (maximum) is achieved when the INA821 uses G = 1 without RG connected. In

this case, gain drift is limited by the slight mismatch of the temperature coefficient of the integrated 10-kΩ

resistors in the differential amplifier (A3). At gains greater than 1, gain drift increases as a result of the individual

drift of the 24.7-kΩ resistors in the feedback of A1 and A2 relative to the drift of the external gain resistor (RG.)

The low temperature coefficient of the internal feedback resistors significantly improves the overall temperature

stability of applications using gains greater than 1 V/V over alternate options.

Low resistor values required for high gain make wiring resistance an important consideration. Sockets add to the

wiring resistance and contribute additional gain error (such as a possible unstable gain error) at gains of

approximately 100 or greater. To maintain stability, avoid parasitic capacitance of more than a few picofarads at

RG connections. Careful matching of any parasitics on the RG pins maintains optimal CMRR over frequency; see

Figure 17.

8.3.2 EMI Rejection

Texas Instruments developed a method to accurately measure the immunity of an amplifier over a broad

frequency spectrum extending from 10 MHz to 6 GHz. This method uses an EMI rejection ratio (EMIRR) to

quantify the ability of the INA821 to reject EMI. The offset resulting from an input EMI signal is calculated using

Equation 2:

where

•

VRF_PEAK is the peak amplitude of the input EMI signal.

(2)

Figure 56 and Figure 57 show the INA821 EMIRR graph for differential and common-mode EMI rejection across

this frequency range. Table 3 lists the EMIRR values for the INA821 at frequencies commonly encountered in

real-world applications. Applications listed in Table 3 are centered on or operated near the particular frequency

shown. Depending on the end-system requirements, additional EMI filters may be required near the signal inputs

of the system. Incorporating known good practices such as using short traces, low-pass filters, and damping

resistors combined with parallel and shielded signal routing may be required.

Figure 56. Common-Mode EMIRR Testing

Figure 57. Differential Mode EMIRR Testing

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