THS4532_16 Datasheet, PDF (30/58 Pages) Texas Instruments – Ultra Low Power, Rail-to-Rail Output, Fully-Differential Amplifier

THS4532
SLOS829A – FEBRUARY 2013 – REVISED JULY 2015
www.ti.com
Table 5. Load Component Values For 1:1 Differential to Single-Ended Output Transformer⁽¹⁾
R_L
100 Ω
200 Ω
500 Ω
1 kΩ
2 kΩ
R_O
25 Ω
84.5 Ω
237 Ω
487 Ω
976 Ω
R_OT
open
71.5 Ω
56.2 Ω
52.3 Ω
51.1 Ω
ATTEN (dB)
6
16.7
25.6
31.8
38
(1) The total load includes 50-Ω termination by the test equipment. Components are chosen to achieve load and 50-Ω line termination
through a 1:1 transformer. Resistor values shown are closest standard values so loads are approximate.
Because of the voltage divider on the output formed by the load component values, the output of the amplifier is
attenuated. The column ATTEN in Table 5 shows the attenuation expected from the resistor divider. When using
a transformer at the output as shown in Figure 72, the signal has slightly more loss because of transformer
insertion loss, and these numbers are approximate. The standard output load used for most tests is 2 kΩ with
associated 38 dB of loss.
9.1.1 Frequency Response and Output Impedance
The circuit shown in Figure 72 is used to measure the frequency response of the 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 AC coupled. R_IT and R_G are selected to impedance match to 50 Ω and maintain
the proper gain. To balance the amplifier, a 49.9-Ω resistor and blocking capacitor to ground is inserted across
R_IT on the alternate input.
The output is routed to the input of the network analyzer through 50-Ω coax. For a 2k load, 38 dB is added to the
measurement to refer back to the output of the amplifier according to Table 5.
For output impedance, the signal is injected at V_OUT with V_IN left open. The voltage drop across the 2x R_O
resistors is measured with a high impedance differential probe and used to calculate the impedance into the
output of the amplifier.
9.1.2 Distortion
At 1 MHz and above, the circuit shown in Figure 72 is used to measure harmonic, intermodulation distortion, and
output impedance of the amplifier.
A signal generator is used as the signal source and the output is measured with a spectrum analyzer. The output
impedance of the signal generator is 50 Ω and is AC coupled. R_IT and R_G are chosen to impedance match to 50
Ω and maintain the proper gain. To balance the amplifier, a 0.22-µF capacitor and 49.9-Ω resistor to ground is
inserted across R_IT on the alternate input. A low-pass filter is inserted in series with the input to reduce
harmonics generated by the signal source. The level of the fundamental is measured and then a high-pass filter
is inserted at the output to reduce the fundamental so it does not generate distortion in the input of the spectrum
analyzer.
Distortion in the audio band is measured using an audio analyzer. Refer to the Audio Performance section for
details.
9.1.3 Slew Rate, Transient Response, Settling Time, Overdrive, Output Voltage, and Turnon and Turnoff
Time
The circuit shown in Figure 73 is used to measure slew rate, transient response, settling time, overdrive
recovery, and output voltage swing. Turnon and turnoff times are measured with 50-Ω input termination on the
PD input, by replacing the 0.22-µF capacitor with 49.9-Ω resistor.
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