THS4521 datasheet(22/51 Pages) TI

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8.4 Differential Amplifier Noise Measurements

To extract the input-referred noise terms from the total output noise, a measurement of the differential output

noise is required under two external conditions to emphasize the different noise terms. A high-gain, low resistor

value condition is used to emphasize the differential input voltage noise and a higher RF at low gains is used to

emphasize the two input current noise terms. The differential output noise must be converted to single-ended

with added gain before being measured by a spectrum analyzer. At low frequencies, a zero 1/f noise, high-gain,

differential to single-ended instrumentation amplifier (such as the INA188) is used. At higher frequencies, a

differential to single-ended balun is used to drive into a high-gain, low-noise, op amp (such as the LMH6629). In

this case, the THS4561 outputs drive 25-Ω resistors into a 1:1 balun where the balun output is terminated single-

endedly at the LMH6629 input with 50 Ω. This termination provides a modest 6-dB insertion loss for the

THS4561 differential output noise that is then followed by a 40-dB gain setting in the very wideband LMH6629.

8.5 Balanced Split-Supply Versus Single-Supply Characterization

Although most end applications use a single-supply implementation, most characterizations are done on a

bipolar balanced supply. Using a bipolar balanced supply keeps the I/O common-mode inputs near midsupply

and provides the most output swing with no DC bias currents for level shifting. These characterizations include

the frequency response, harmonic distortion, and noise plots. The time domain plots are in some cases done

through single-supply characterization to obtain the correct movement of the input common-mode voltage.

8.6 Simulated Characterization Curves

In some cases, a characteristic curve can only be generated through simulation. A good example of this scenario

is the output balance plot of Figure 7-51. This plot shows the best-case output balance (output differential signal

versus output common-mode signal) using exact matching of the external resistors in simulation using a single-

ended input to differential output configuration. The actual output balance is set by resistor mismatch at low

frequencies but intersects and follows the high-frequency portion of Figure 7-51 at higher frequencies.

The remaining simulated plots include:

• AOL gain and phase; see Figure 7-41.

• CMRR and PSRR vs frequency; see Figure 7-52.

THS4561

SBOS874C – AUGUST 2017 – REVISED DECEMBER 2020

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THS4521 Datasheet(PDF) 22 Page - Texas Instruments