THS4541-Q1 Datasheet, PDF (37/60 Pages) Texas Instruments – 850-MHz Fully Differential Amplifier

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THS4541-Q1
SLOS930A – NOVEMBER 2015 – REVISED NOVEMBER 2015
Device Functional Modes (continued)
grounded for DC-coupled, bipolar-input applications. This configuration gives a balanced differential output if the
source is swinging around ground. If the source swings from ground to some positive voltage, grounding Rg2
gives a unipolar output differential swing from both outputs at Vocm (when the input is at ground) to one polarity
of swing. Biasing Rg2 to an expected midpoint for the input signal creates a differential output swing around
Vocm.
One significant consideration for a DC-coupled input is that Vocm sets up a common-mode bias current from the
output back through Rf and Rg to the source on both sides of the feedback. Without input balancing networks,
the source must sink or source this DC current. After the input signal range and biasing on the other Rg element
is set, check that the voltage divider from Vocm to Vin through Rf and Rg (and possibly Rs) establishes an input
Vicm at the device input pins that is in range. If the average source is at ground, the negative rail input stage for
the THS4541-Q1 is in range for applications using a single positive supply and a positive output Vocm setting
because this DC current lifts the average FDA input summing junctions up off of ground to a positive voltage (the
average of the V+ and V– input pin voltages on the FDA).
9.4.1.3 Resistor Design Equations for the Single-Ended to Differential Configuration of the FDA
The design equations for setting the resistors around an FDA to convert from a single-ended input signal to
differential output can be approached from several directions. Here, several critical assumptions are made to
simplify the results:
• The feedback resistors are selected first and set equal on the two sides.
• The DC and AC impedances from the summing junctions back to the signal source and ground (or a bias
voltage on the nonsignal input side) are set equal to retain feedback divider balance on each side of the FDA
Both of these assumptions are typical and aimed to delivering the best dynamic range through the FDA signal
path.
After the feedback resistor values are chosen, the aim is to solve for the Rt (a termination resistor to ground on
the signal input side), Rg1 (the input gain resistor for the signal path), and Rg2 (the matching gain resistor on the
nonsignal input side); see Figure 61 and Figure 63. 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 Rt element (as shown in Figure 61) has the advantage of removing any DC currents in the
feedback path from the output Vocm to ground.
Earlier approaches to the solutions for Rt and Rg1 (when the input must be matched to a source impedance, Rs)
follow an iterative approach. This complexity arises from the active input impedance at the Rg1 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 Rg2 element. A more
recent solution is shown as Equation 7, where a quadratic in Rt 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 Rf value.
2. The target voltage gain (Av) from the input of Rt to the differential output voltage.
3. The desired input impedance at the junction of Rt and Rg1 to match Rs.
Solving this quadratic for Rt starts the solution sequence, as shown in Equation 7.
Rt² Rt
2Rs^§¨© 2Rf
Rs
2
Av²
·
¸¹
2RfRs²Av
0
2Rf 2 Av RsAv(4 Av) 2Rf 2 Av RsAv(4 Av)
(7)
Being a quadratic, there are limits to the range of solutions. Specifically, after Rf and Rs are chosen, there is
physically a maximum gain beyond which Equation 7 starts to solve for negative Rt values (if input matching is a
requirement). With Rf selected, use Equation 8 to verify that the maximum gain is greater than the desired gain.
Av _max
ª
Rf
Rs
«
2
‡
«1
«
«¬
4 ^Rf
º
»
1
Rs _»
( ^Rf
Rs
2)²
»
»¼
(8)
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