JAJSMA5C June 2021 – March 2022 BUF802
PRODUCTION DATA
The specified input referred voltage noise of the BUF802, as shown in Section 6.5, is 2.3 nV/√Hz. The total input referred RMS noise in a bandwidth of 1 GHz is given by the following equation:
1.22 = Brickwall correction factor. Detailed calculations can be found on TI Precision Labs – Op Amps: Noise – Spectral Density.
Total input refered spot noise as a function of frequency is shown in Figure 9-3. Assuming the oscilloscope has 8 divisions on the screen and a highest resolution of 1 mV, the full-scale reading is 8 mVPP or 2.82 mVRMS. Thus, the SNR of the front-end amplifier stage at the highest-resolution setting is:
The parasitic input capacitance of BUF802 (IN pin) is 2.4 pF. At 1 GHz this parasitic capacitance reduces down to an impedance of 66.3 Ω. Thus, the net input impedance as seen by the signal at the input is the following:
CL Mode of BUF802 simplifies this design for achieving a flat frequency response from DC till the frequency of interest (1 GHz in this case). To achieve a flat response, the following two conditions have to be met:
α is the input attenuation factor and β is the inverse of the non-inverting gain of the precision amplifier. G is the DC 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 frequency 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.
fLF is the pole resulting from the gain bandwidth of the precision amplifier (OPA140), the Auxiliary Path bandwidth and other parasitic capacitance of the resistor network.
Where GBW is the gain bandwidth product of the precision amplifier (OPA140) = 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:
Where CINPA is the common mode input capacitance of the precision amplifier, OPA140 in this case.
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 or may not be trimmed along with RPOT in the final production flow.