Figure 9-5 and Figure 9-6 provide a graphical comparison of the described noise densities versus different gain

settings. Each of the contributors are separately showcased in the graphs. As expected, lower feedback

resistors (in this case, 2 kΩ) show that the dominant factor of the total output noise is the intrinsic voltage noise

of the FDA (at gains > 2). For smaller gain settings, the thermal noise of the feedback resistors is dominating.

Gain (V/V)

1

10

10

í9

10

í8

10

í7

0.1

100

10

í6

Figure 9-5. Calculated Noise Densities vs Gain

Settings

Gain (V/V)

1

10

10

í9

10

í8

10

í7

10

í6

0.1

100

Figure 9-6. Calculated Noise Densities vs Gain

Settings

The advancement of the THP210 can be seen at higher feedback resistors (in this case 10 kΩ). Many FDAs

exhibit an input current noise density in the range of some pA/√Hz that, in cases for higher feedback resistors,

dictate the noise behavior. As a result of the superior current noise density of 300 fA/√Hz of the THP210, the

overall output noise is mainly dominated by the thermal noise of the resistors (here, up to gains of approximately

15).

The total output voltage noise density is important when using FDAs as ADC input driver stages. To evaluate the

compatibility between the input driver and the ADC from a noise perspective, compare the calculated RMS

output noise of the FDA with the least-significant bit (LSB) of the desired ADC application, in respect to the

effective number of bits (ENOB). Section 9.2.2 shows measurements of the THP210 in combination with state-

of-the-art SAR ADCs, and indicates the performance that is achieved.

9.1.4 Mismatch of External Feedback Network

The common-mode rejection ratio (CMRR) is one of the key elements when designing with fully differential

amplifiers. Although FDAs are designed to provide the best CMRR performance, poor selection of external gain

setting resistors, as well as careless board layout techniques, significantly degrade CMRR performance.

the previous section, the mismatch of the external resistor network primarily contributes to the dc error.

Generally, a resistor mismatch of 0.1% and a ratio of 1 V/V results in a CMRR of 60 dB. The natural degradation

of the external resistor network is minimized by the following guidelines:

• Consider input impedance matching, as shown in the Input impedance matching with fully differential

amplifiers technical brief.

• Follow layout guidelines, as provided in Section 11.1.

• Use compensation techniques, as described in the Improving PSRR and CMRR in Fully Differential

Amplifiers application report

Despite the mismatch of the external feedback network, the internal common-mode feedback amplifier regulates

the outputs to remain balanced in amplitude and remain 180° out of phase. The output balance performance

stays unaffected by the CMRR degradation.

www.ti.com

THP210

SBOS932B – FEBRUARY 2020 – REVISED OCTOBER 2020

Copyright © 2020 Texas Instruments Incorporated

Submit Document Feedback

23

Product Folder Links: THP210