LMH3401 Datasheet, PDF (23/48 Pages) Texas Instruments – LMH3401 7-GHz, Ultra-Wideband, Fixed-Gain, Fully-Differential Amplifier

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LMH3401
SBOS695A – AUGUST 2014 – REVISED DECEMBER 2014
Feature Description (continued)
9.3.2.1 Resistor Design Equations for Single-to-Differential Applications
Even though the resistors for the LMH3401 are on-chip, being familiar with the FDA resistor selection criteria is
still important. The design equations for setting the resistors around an FDA to convert from a single-ended input
signal to a differential output can be approached in several ways. In this section, several critical assumptions are
made to simplify the results:
• The feedback resistors are selected first and set to be equal on the two sides of the device.
• The dc and ac impedances from the summing junctions back to the signal source and ground (or a bias
voltage on the non-signal input side) are set as equal to retain the feedback divider balance on each side of
the FDA.
Both of these assumptions are typical and aimed to deliver the best dynamic range through the FDA signal path.
After the feedback resistor values are chosen, the aim is to solve for R_T (a termination resistor to ground on the
signal input side), R_G1 (the input gain resistor for the signal path), and R_G2 (the matching gain resistor on the
non-signal input side), as shown in Figure 55 (this example uses the THS4541, an external resistor FDA). The
same resistor solutions can be applied to either ac- or dc-coupled paths. Adding blocking capacitors in the input-
signal chain is a simple option. Adding these blocking capacitors after the R_T element (as shown in Figure 55)
has the advantage of removing any dc currents in the feedback path from the output V_OCM to ground.
50- Input Match Gain of
2 V/V from R_T Single-Ended
Source to Differential Output
THS4541 Wideband,
Fully-Differential Amplifier
RF1
402
50- 
Source
C1
100 nF
RG1
191
RT
60.2
VOCM
RG2
221
C2
100 nF
VCC
±
+
FDA
±
+
PD
VCC
RF2
402
Output
RLOAD
500
Measurement
Point
Figure 55. AC-Coupled, Single-Ended Source to a Differential Gain of a 2-V/V Test Circuit
Most FDA amplifiers use external resistors and have complete flexibility in the selected R_F, just like the THS4541
does in Figure 55, however the LMH3401 has on-chip feedback resistors that are fixed at 200 Ω. The equations
used in this section still apply, and an external resistance can be added to the on-chip R_G resistors.
After the feedback resistor values are chosen, the aim is to solve for R_T (a termination resistor to ground on the
signal input side), R_G1 (the input gain resistor for the signal path), and R_G2 (the matching gain resistor on the
non-signal input side). The same resistor solutions can be applied to either ac- or dc-coupled paths. Adding
blocking capacitors in the input-signal chain is a simple option. Adding these blocking capacitors after the R_T
element has the advantage of removing any dc currents in the feedback path from the output V_OCM to ground.
Earlier approaches to the solutions for R_T and R_G1 (when the input must be matched to a source impedance, R_S)
follow an iterative approach. This complexity arises from the active input impedance at the R_G1 input. When the
FDA is used to convert a single-ended signal to differential, the common-mode input voltage at the FDA inputs
must move with the input signal to generate the inverted output signal as a current in the R_G2 element. A more
recent solution is illustrated in Equation 2, where a quadratic in R_T can be solved for an exact required value.
This quadratic emerges from the simultaneous solution for a matched input impedance and target gain. The only
inputs required are:
1. The selected R_F value.
2. The target voltage gain (A_V) from the input of R_T to the differential output voltage.
3. The desired input impedance at the junction of R_T and R_G1 to match R_S.
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