THS4532_16 Datasheet, PDF (46/58 Pages) Texas Instruments – Ultra Low Power, Rail-to-Rail Output, Fully-Differential Amplifier

THS4532
SLOS829A – FEBRUARY 2013 – REVISED JULY 2015
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Systems Examples (continued)
After the feedback resistor values are chosen, the aim is to solve for the 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
nonsignal input side); see Figure 98 and Figure 99. 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 98) 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 shown as Equation 8, where a quadratic in R_T can be solved for an exact value. This quadratic
emerges from the simultaneous solution for a matched input impedance and target gain. The only inputs required
are:
• The selected R_F value.
• The target voltage gain (A_v) from the input of R_T to the differential output voltage.
• The desired input impedance at the junction of R_T and R_G1 to match R_S.
Solving this quadratic for R_T starts the solution sequence, as shown in Equation 8:
R
2
T
-
R
T
2R _S(2R _F
2R _F(2 + A_V ) -
+
R_S
2
A
2
V
)
R _SA_V(4 +
A_V )
-
2R _F(2 +
2R _F R_S2 A_V
A_V ) - R _SA_V(4 + A_V)
=0
(8)
Being a quadratic, there are limits to the range of solutions. Specifically, after R_F and R_S are chosen, there is
physically a maximum gain beyond which Equation 8 starts to solve for negative R_T values (if input matching is a
requirement). With R_F selected, use Equation 9 to verify that the maximum gain is greater than the desired gain.
é
ù
ê
A _V(MAX)
=
æ
ççè
R
R
F
S
ö
- 2÷÷ø ´
ê
êê_{1 +}
ê
êë
1
+
4R^F
R_S
æ
ççè
R
R
F
S
ö²
- 2÷÷ø
ú
ú
ú
ú
ú
úû
(9)
If the achievable A_V(MAX) is less than desired, increase the R_F value. After R_T is derived from Equation 8, the R_G1
element is given by Equation 10:
R _G1
=
2R^F
A_V
1+
- R_S
R_S
R_T
(10)
Then, the simplest approach is to use a single R_G2 = R_T || R_S + R_G1 on the nonsignal input side. Often, this
approach is shown as the separate R_G1 and R_S elements. Using these separate elements provides a better
divider match on the two feedback paths, but a single R_G2 is often acceptable. A direct solution for R_G2 is given
as Equation 11:
2R^F
R _G2
=
A_V
1+
R_S
R_T
(11)
This design proceeds from a target input impedance matched to R_S, signal gain A_v from the matched input to the
differential output voltage, and a selected R_F value. The nominal R_F value chosen for the THS5432 family
characterization is 2 kΩ. As discussed previously, going lower improves noise and phase margin, but reduces the
total output load impedance possibly degrading harmonic distortion. Going higher increases the output noise, and
might reduce the loop-phase margin because of the feedback pole to the input capacitance, but reduces the total
loading on the outputs.
46
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