THS4551 Datasheet, PDF (32/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
www.ti.com
As a third example of arriving at the input pin voltage swings, use the design of Figure 93 (the ADC3241 design).
Thevenize the source to just one input resistor to get an expression for the input V_ICM in terms of the input
voltage to be derived. Figure 72 shows the gain of 5 V/V, dc-coupled, matched input impedance, single-ended to
differential circuit of Figure 93 with both sides reduced to a single input resistor. In Figure 93, the design operates
on a single 3.3-V supply with an output V_OCM equal to 0.95 V to directly connect to TI’s line of low-power
ADC3xxx series of 12- and 14-bit ADCs. This family accepts a 2-V_PP maximum differential voltage, which (at the
input-terminating resistor of Figure 93) is a ±0.2-V swing. Going back to the source through the matching resistor
is then a ±0.4-V source swing. Thevenizing that source with the R_T element provides the ±0.217 V shown in
Figure 72 and the total R2 as the sum of R_G1 and 50 Ω || 59 Ω.
VS+
VS-
VOCM
+
3.3 V
±
0V ⁺
±
+
0.95 V
±
Thevinized
Source
VTHEV
±0.217 V
R2
215
THS4551 Wideband,
Fully Differential Amplifier
VOCM
RF1
1k
VS+
±
+
FDA
±
+
PD
VOUT
±1-V Differential Output
Centered on 0.95 VCM
RL
1k
R2
215
VS- ^VS+
RF2
1k
Copyright © 2016, Texas Instruments Incorporated
Figure 72. Input V_ICM Analysis Circuit From the Design of Figure 93
For an input signal (V_THEV) that swings around ground as ±V_THEV, the input pins are within a range given by
Equation 7, which is a superposition of the output V_OCM divided back to the input nodes and half of the input
±V_THEV signal.
V_ICM
V_OCM
R_G
R_G R_F
r
V_THEV
2
u ^RF
R_F R_G
(7)
Using the values from the design of Figure 72, the computed input range for the THS4551 input pins is V_ICM =
0.168 V ± 0.89 mV or 0.079 V to 0.257 V at the input pins. These values are well within range for the negative
rail input available in the THS4551.
A simpler approach to arriving at the input common mode range for this DC coupled single supply design would
be to take the output voltage swing range on the lower side (non – signal input side) and simply divide it back
through its resistor divider to ground on that side.
32
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