THS4561 Datasheet, PDF (14/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

THS4561
SBOS874 – AUGUST 2017
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8 Parameter Measurement Information
8.1 Example Characterization Circuits
The THS4561 offers the advantages of a fully differential amplifier (FDA) design with the trimmed input offset
voltage and low drift of a precision op amp. The FDA is a flexible device where the main aim is to provide a
purely differential output signal centered on a user-configurable common-mode voltage usually matched to the
input common-mode voltage required by an analog-to-digital converter (ADC) following this stage. The primary
options revolve around the choices of single-ended or differential inputs, ac-coupled or dc-coupled signal paths,
gain targets, and resistor value selections. The characterizations described in this section focus on single-ended
input to differential output designs as the more challenging application requirement. Differential sources are
supported and are simple to implement and analyze.
The characterization circuits are typically operated with a single-ended, matched, 50-Ω, input termination to a
differential output at the FDA output pins because most lab equipment is single-ended. The FDA differential
output is then translated back to single-ended through a variety of baluns (or transformers) depending on the test
and frequency range. DC-coupled step response testing uses two 50-Ω scope inputs with trace math. Single-
supply operation is most common in end equipment designs. However, using split balanced supplies allows
simple ground referenced testing without adding further blocking capacitors in the signal path beyond those
capacitors already within the test equipment. The starting point for any single-ended input to differential output
measurements (such as any of the frequency response curves) is shown in Figure 1.
Network
Analyzer,
50- Source
Impedance
50- Input Match,
Gain of 1 V/V from RT,
Single-Ended Source to
Differential Output
RG1
1.5 k
RT1
52.3
THS4561 Wideband,
Fully Differential Amplifier
RF1
1.5 k
VS+
VOCM
±
+
FDA
±
+
PD
VOPP
31.8-dB
Insertion Loss
from VOPP to a
50- Load
RO1
487
RO2
487
50- Termination
RS1
50
RG2
1.5 k
RT2
52.3
VS± ^VS+
RF2
1.5 k
1-k
Differential
Load
ADTL1-4-75+
RM
52.3
N1
50-
Single-Ended
N2
Source
Network
Analyzer,
50- Load
Copyright © 2017, Texas Instruments Incorporated
Figure 1. Single-Ended Source to a Differential Gain of a 1-V/V Test Circuit
Most characterization plots fix the R_F (R_F1 = R_F2) value at 1.5 kΩ, as shown in Figure 1. This element value is
flexible in application, but 1.5 kΩ provides a good compromise for the parasitic issues linked to this value,
specifically:
• Added output loading: The FDA functions similarly to an inverting op amp design with feedback resistors
appearing as an added load across the outputs (the approximate total differential load in Figure 1 is 1.5 kΩ ||
1 kΩ = 857 Ω). The 1.5-kΩ value reduces the power dissipated in the feedback networks.
• Noise contributions resulting from resistor values: These contributions are both the 4kTR_F terms and the
current noise times the R_F value to the output (see Noise Analysis ).
• Parasitic feedback pole at the input summing nodes: this pole is created by the feedback resistor (R_F) value
and the 2.4-pF differential input capacitance (as well as any board layout parasitic) and introduces a zero in
the noise gain, which decreases the phase margin in most situations. This effect must be managed for best
frequency response flatness or step response overshoot.
The frequency domain characterization curves start with the selections of Figure 1. Some of the features in this
test circuit include:
• The elements on the non-signal input side match the signal input resistors. This feature closely matches the
divider networks on each side of the FDA. The three resistors on the non-signal input side can be replaced by
a single resistor to ground using a standard value of 1.5 kΩ with some loss in gain balancing between the two
sides; see ).
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