THS4531A_15 Datasheet, PDF (31/59 Pages) Texas Instruments – THS4531A Ultra Low-Power, Rail-to-Rail Output, Fully Differential Amplifier

www.ti.com
THS4531A
SLOS823B – DECEMBER 2012 – REVISED JUNE 2015
9.1.6 Balance Error
The circuit shown in Figure 75 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. R_IT and R_G 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 R_IT on the alternate input.
The output is measured using a high impedance differential probe at the summing junction of the two R_O
resistors, with respect to ground.
V_IN+
Input From 50-Ω
Test Equipment
R_G
R_F
R_O
R_IT
Cal Diff Probe
V_IN-
No Connection
49.9 Ω
Installed to Balance
Amplifier's Feedback Networks
0.22 µF
R_G
R_IT
V_S+
+
VOCM
±
V_OUT±
THS4531A
V_S-
V_OUT+
R_O
R_F
Measure With
Diff Probe Here
Figure 75. Balance Error Test Circuit
9.1.7 Single-Supply Operation
To facilitate testing with common lab equipment, the THS4531A 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 THS4531A is designed for the nominal value of R_F 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 R_F = R_G = 2 kΩ, R_G to ground, and V_OUT+ = 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 76 shows the small signal frequency response for gain of 1 with R_F and R_G equal to 2 kΩ, 10 kΩ, and 100
kΩ. The test was done with R_L = 2 kΩ. Because of loading effects of R_L, 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.
Copyright © 2012–2015, Texas Instruments Incorporated
Product Folder Links: THS4531A
Submit Documentation Feedback
31