INA821 datasheet(19/36 Pages) TI1

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110

10M

100M

10G

Frequency (Hz)

C001

110

10M

100M

10G

Frequency (Hz)

C001

EMIRR (dB)

RF _ PEAK

100 mV

INA821

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SBOS893 – AUGUST 2018

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8.3.1.1 Gain Drift

The stability and temperature drift of the external gain setting resistor (RG) 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 important. 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 ensure 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 52 and Figure 53 show the INA821 EMIRR graph for differential and common-mode EMI rejection across

this frequency range. Table 2 shows the EMIRR values for the INA821 at frequencies encountered in real-world

applications. Applications listed in Table 2 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, as well as incorporating known good practices such as using short traces, low-pass filters, and damping

resistors combined with parallel and shielded signal routing.

Figure 52. Common-Mode EMIRR Testing

Figure 53. Differential Mode EMIRR Testing

Table 2. INA821 EMIRR for Frequencies of Interest

FREQUENCY

APPLICATION OR ALLOCATION

DIFFERENTIAL

EMIRR

COMMON-MODE

EMIRR

400 MHz

Mobile radio, mobile satellite, space operation, weather, radar, ultrahigh-

frequency (UHF) applications

48 dB

87 dB

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