THS4551 Datasheet, PDF (21/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

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THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
8 Parameter Measurement Information
8.1 Example Characterization Circuits
The THS4551 offers the advantages of a fully differential amplifier (FDA) design with the trimmed input offset
voltage and very low drift of a precision op amp. The FDA is an extremely 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 can certainly be supported and are often simpler to both 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 used 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 61.
Network
Analyzer,
50- Source
Impedance
50- Input Match,
Gain of 1 V/V from RT,
Single-Ended Source to
Differential Output
RG1
1k
RT1
52.3
THS4551 Wideband,
Fully Differential Amplifier
RF1
1k
VS+
VOCM
±
+
FDA
±
+
PD
VOPP
29.5-dB
Insertion Loss
from VOPP to a
50- Load
RO1
487
RO2
487
50- Termination
RS1
50
RG2
1k
RT2
52.3
VS- ^VS+
RF2
1k
1-k
Differential
Load
ADTL1-4-75+
RM
52.3
N1
50-
Single-Ended
N2
Source
Network
Analyzer,
50- Load
Copyright © 2016, Texas Instruments Incorporated
Figure 61. 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 kΩ, as shown in Figure 61. This element value is
completely flexible in application, but 1 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 both feedback resistors
appearing as an added load across the outputs (the approximate total differential load in Figure 61 is
1 kΩ || 2 kΩ = 667 Ω). The 1-kΩ value also 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 the Noise Analysis section).
• Parasitic feedback pole at the input summing nodes: this pole is created by the feedback resistor (R_F) value
and the 1.2-pF differential input capacitance (as well as any board layout parasitic) and introduces a zero in
the noise gain, thus decreasing the phase margin in most situations. This effect must be managed for best
frequency response flatness or step response overshoot. Internal 0.6-pF feedback capacitors on each side
combine with these external feedback resistors to introduce a zero in the noise gain, thereby reducing the
effect of the feedback pole to the differential input capacitance.
Copyright © 2016, Texas Instruments Incorporated
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