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THS4532_15 Datasheet, PDF (33/58 Pages) Texas Instruments – THS4532 Ultra Low Power, Rail-to-Rail Output, Fully-Differential Amplifier
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THS4532
SLOS829A – FEBRUARY 2013 – REVISED JULY 2015
9.1.6 Balance Error
The circuit shown in Figure 77 is used to measure the balance error of the main differential amplifier. A network
analyzer is used as the signal source and the measurement device. The output impedance of the network
analyzer is 50 Ω and is DC coupled. RIT and RG are chosen to impedance match to 50 Ω and maintain the
proper gain. To balance the amplifier, a 49.9-Ω resistor to ground is inserted across RIT on the alternate input.
The output is measured using a high impedance differential probe at the summing junction of the two RO
resistors, with respect to ground.
VIN+
Input From 50-Ω
Test Equipment
RG
RF
RO
RIT
Cal Diff Probe
VIN-
No Connection
49.9 Ω
Installed to Balance
Amplifier's Feedback Networks
0.22 µF
RG
RIT
VS+
+
VOCM
±
VOUT±
THS4532
VS-
VOUT+
RO
RF
Measure With
Diff Probe Here
Figure 77. Balance Error Test Circuit
9.1.7 Single-Supply Operation
To facilitate testing with common lab equipment, the THS5432 EVM is built to allow for split-supply operation and
most of the data presented in this data sheet was taken with split-supply power inputs. The device is designed
for use with single-supply power operation and can easily be used with single-supply power without degrading
the performance. The only requirement is to bias the device properly and the specifications in this data sheet are
given for single supply operation.
9.1.8 Low-Power Applications and the Effects of Resistor Values on Bandwidth
The THS5432 is designed for the nominal value of RF to be 2 kΩ. This gives excellent distortion performance,
maximum bandwidth, best flatness, and best pulse response. It also loads the amplifier. For example; in gain of 1
with RF = RG = 2 kΩ, RG to ground, and VOUT+ = 4 V, 1 mA of current will flow through the feedback path to
ground. In low power applications, reducing this current is desirable by increasing the gain setting resistors
values. Using larger value gain resistors has three primary side effects (other than lower power) because of the
interaction with the device and PCB parasitic capacitance:
• Lowers the bandwidth.
• Lowers the phase margin.
– This causes peaking in the frequency response.
– This also causes overshoot and ringing in the pulse response.
• Increases the output noise.
Figure 78 shows the small signal frequency response for gain of 1 with RF and RG equal to 2 kΩ, 10 kΩ, and 100
kΩ. The test was done with RL = 2 kΩ. Because of loading effects of RL, lower values may reduce the peaking,
but higher values will not have a significant effect.
As expected, larger value gain resistors cause lower bandwidth and peaking in the response (peaking in
frequency response is synonymous with overshoot and ringing in pulse response). These effects are caused by
the feedback pole created by the summing-junction capacitance and these larger Rf values.
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