THS4561 Datasheet, PDF (17/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

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THS4561
SBOS874 – AUGUST 2017
Output Common-Mode Measurements (continued)
In Figure 5, the differential path is terminated back to ground on the two 1.5-kΩ input resistors and the V_OCM
control input is driven from a 50-Ω matched source for the frequency response and step response curves of and .
The outputs are summed to a center point (to obtain the average, or common-mode, output) through two 100-Ω
resistors. These 100-Ω resistors form an equivalent 50-Ω source to the common-mode output for measurements.
This common-mode test circuit is available as a TINA-TI™ simulation file. illustrates the common-mode output
noise measurements with a ground on the VOCM input pin or with the VOCM input pin floating. The higher noise
in for a floated input can be reduced by including a capacitor to ground at the VOCM control input pin.
8.4 Differential Amplifier Noise Measurements
To extract out 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 R_F 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 split
balanced supply. Using a split 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 via 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 . This plot shows the best-case output balance (output differential signal versus
output common-mode signal) using exact matching on 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 .
The remaining simulated plots include:
• A_OL gain and phase; see .
• Large- and small-signal settling times; see and .
• Closed-loop output impedance versus frequency; see .
• CMRR vs frequency; see .
• PSRR vs frequency and output common-mode voltage;see , , and .
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