BUF802 datasheet(20/32 Pages) TI

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α is the input attenuation factor of the precision amplifier, whereas β is the inverse of its non-inverting gain. G

is the gain of the Main path of the BUF802. Since G can vary from device-to-device, trimming either α or β is

recommended to achieve a flat response. In Figure 9-1, β may be trimmed using the RPOT. Since G is ≈1 V/V

and α is 1/5 (200 kΩ / (200 kΩ + 800 kΩ)), RPOT should be trimmed so that β = 1/5. For the β network, it is

recommended to use resistors which are an order of magnitude of resistance lower than the resistors used in the

α network. Therefore β resistor values of 80 kΩ and ≈20 kΩ have been chosen.

fHF is the pole resulting from the 330 pF series capacitor and the 10 MΩ resistor on the In_Bias pin.

fHF = 1/(2 × pi × R × C) = 1/(2 × 3.14 × 10 MΩ × 330 pF) = 48 Hz

(3)

fLF is the pole resulting from the gain bandwidth of the precision amplifier, the Auxiliary path bandwidth and other

parasitic capacitance of the BUF802.

fLF = GBW × GAUX × β = 440 kHz

(4)

Where GBW is the gain bandwidth product of the precision amplifier = 11 MHz. GAUX is the gain from In_Aux to

OUT = 0.2 V/V. 1/β is the external non-inverting gain set for the precision amplifier = 5 V/V.

Based on the above value of fHF and fLF, the required condition of fHF<< fLF is met. CF, connected across the

precision amplifier, is required to compensate for the parasitic capacitance and to make the overall poles and

zeros cancel each other. The value of CF can be found by using the following equation:

CF = CINPA × ((G × Rα2 / Rβ2) – 1)).

(5)

Where CINPA is the common mode input capacitance of the precision amplifier.

Plugging in the value of these components arrives at CF = 56 pF. In the final system, based on the quality of the

flat band response needed, CF may be trimmed along with RPOT in the final production flow.

9.2.1.3 Application Curves

Frequency (Hz)

-1

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

0.8

100

10k

100k

Figure 9-3. Frequency Response in Cross-Over

Frequency Region

Frequency (Hz)

-18

-15

-12

-9

-6

-3

10M

100M

10G

Figure 9-4. 2 GHz Frequency Response

BUF802

SBOS998 – JUNE 2021

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