THS4521 Datasheet, PDF (39/65 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

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THS4521, THS4522, THS4524
SBOS458H – DECEMBER 2008 – REVISED JUNE 2015
8.4.2 Differential-Input to Differential-Output Operation
In many ways, this method is a much simpler way to operate the FDA from a design-equations perspective.
Again, assuming the two sides of the circuit are balanced with equal R_F and R_G elements, the differential input
impedance is now just the sum of the two R_G elements to a differential inverting summing junction. In these
designs, the input common-mode voltage at the summing junctions does not move with the signal, but must be
dc biased in the allowable range for the input pins with consideration given to the voltage headroom required
from each supply. Slightly different considerations apply to ac- or dc-coupled, differential-in to differential-out
designs, as described in the following sections.
8.4.2.1 AC-Coupled, Differential-Input to Differential-Output Design Issues
There are two typical ways to use the THS452x family with an ac-coupled differential source. In the first method,
the source is differential and can be coupled in through two blocking capacitors. The second method uses either
a single-ended or a differential source and couples in through a transformer (or balun). Figure 77 shows a typical
blocking capacitor approach to a differential input. An optional differential-input termination resistor (R_M) is
included in this design. This R_M element allows the input R_G resistors to be scaled up while still delivering lower
differential input impedance to the source. In this example, the R_G elements sum to show a 500-Ω differential
impedance, while the R_M element combines in parallel to give a net 100-Ω, ac-coupled, differential impedance to
the source. Again, the design proceeds ideally by selecting the R_F element values, then the R_G to set the
differential gain, then an R_M element (if needed) to achieve the target input impedance. Alternatively, the R_M
element can be eliminated, the R_G elements set to the desired input impedance, and R_F set to the get the
differential gain (R_F / R_G).
THS452x Wideband,
Fully-Differential Amplifier
Rf1
1 kΩ
C1
100 nF
Rg1
250 Ω
Downconverter
Differential
Output
C2
100 nF
Vocm
Rm
125 Ω
Rg2
250 Ω
Vcc
–
+
FDA
–
+
PD
Vcc
Rf2
1 kΩ
R1
500 Ω
Output
Measurement
Point
Figure 77. Example Down-Converting Mixer Delivering an AC-Coupled Differential Signal to the THS452x
The dc biasing here is very simple. The output V_OCM is set by the input control voltage; and because there is no
dc-current path for the output common-mode voltage, that dc bias also sets the input pins common-mode
operating points.
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