Datasheet

IN-
OUT-
IN+
OUT+
200 W
+5V
1k W
B0toB5
0.1 Fm
0.1 Fm
From150-
DifferentialSource
W
V
MID2
0.1 Fm
V
MID1
0.1 Fm
LM
(1)
GS
(1)
PD
PGA870
B0toB5
PGA870
www.ti.com SBOS436A –DECEMBER 2009–REVISED FEBRUARY 2011
Differential-to-Differential Operation
Differential operation of PGA870 is shown in Figure 46. In this example, both input pins are connected to a
differential 150-Ω source. The PGA870 is driving a typical 200-Ω load. Both midsupply voltage pins V
MID1
and
V
MID2
are bypassed with a 0.1-μF capacitor. The LATCH MODE, GAIN STROBE, and PD pins are connected to
the power supply using a 1-kΩ pull-up resistor. The PD pin set high powers up the PGA870, while setting the
Latch Mode and the Gain Strobe pins high bypasses the latch mode, allowing instantaneous gain changes as B5
to B0 change.
If a differential signal source is to be dc-coupled to the device, it should have a common-mode voltage that is
within 0.2 V of the midsupply reference. If the input common-mode is greater than 0.2 V from midsupply, then
increased distortion and reduced performance can result.
(1) LM = LATCH MODE pin (pin 1), GS = GAIN STROBE pin (pin 7).
Figure 46. Basic Connections for Fully Differential Operation
Operation with Split Supply ±2.5 V
The PGA870 can be operated using a split ±2.5-V supply. In this case, V
S+
is connected to +2.5 V, and GND
(and any other pin noted to be connected to GND) is connected to –2.5 V. As with any device, what the user
decides to name the levels in the system is irrelevant to the PGA870. In essence, it is simply a level shift of the
power pins and all voltage levels by –2.5 V. With a ±2.5-V power supply, the output common-mode voltage is 0 V
and input and output voltage ranges are symmetrical around 0 V. The power-down and gain control logic input
thresholds all shift to relative to –2.5 V; that is, the logic low threshold of 0.9 V with a single 5-V supply shifts to
1.6 V with ±2.5-V supplies, and the logic high threshold of 2.1 V with a single 5-V supply shifts to –0.4 V with
±2.5-V supplies. Level-shifting the logic signals may require a comparator circuit for each logic signal line.
© 2009–2011, Texas Instruments Incorporated 19
Product Folder Link(s): PGA870