Datasheet

1/2
TLV3502
+2.5 V
+2.5 V
-2.5 V
Logic to PGA870 Pin
V = +2.5 V, V = 2.5 V-
OH OL
2.5-V Input Logic
V = 2.1 V, V = 0.9 V
OH OL
R
R
Bandpass
Filter
and
Bias
PGA870
R
O
R
O
ADC
A
IN-
A
IN+
CM
SNR = 10log
Amp+Filter
= 20log
V
e
O
Filterout
2
2
(
(
V
e
O
Filterout
(
(
with:
e = e
Filterout NAmpout
ENB
PGA870
SBOS436A –DECEMBER 2009–REVISED FEBRUARY 2011 www.ti.com
Figure 47 shows one possible circuit using one channel of the high-speed, 4.5-ns propagation delay TLV3502.
The switching speed of the output logic signal is limited by the propagation delay of the comparator. Using slower
comparators limits the digital logic speed and can thus limit the gain control speed in automatic gain control
applications.
Figure 47. Comparator Circuit to Shift Logic Signals to PGA870 Operated on Split ±2.5-V Supplies
Using the PGA870 with split ±2.5-V supplies allows for an input signal centered around ground and sets the
output common mode of the PGA870 to ground. The ADC Input Common-Mode Voltage Considerations:
DC-Coupled Input section discusses how an output common-mode voltage of 0 V provides less signal
attenuation when using a level-translating resistor network to drive an ADC with a low input common-mode
voltage in dc-coupled applications.
Driving ADCs
The PGA870 is designed and optimized to drive differential input ADCs for the lowest distortion performance.
Figure 48 shows a generic block diagram of the PGA870 driving an ADC. The primary interface circuit between
the amplifier and the ADC is a noise-limiting and anti-aliasing filter that may also provide a means to bias the
signal to the input common-mode voltage required by the ADC. Filters range from single-order real RC poles to
higher-order LC filters, depending on the application requirements. Output resistors (R
O
) are shown on the
amplifier outputs to isolate the amplifier from any capacitive loading presented by the filter as the PGA870
presents a low impedance on its outputs.
Figure 48. Generic ADC Driver Block Diagram
Key points to consider for successfully implementing the PGA870 are described in the following subsections.
SNR Considerations
Depending on the amplitude of the signal and the bandwidth of the filter, the SNR of the amplifier and filter
together can be calculated. Note that the noise from the amplifier is band-limited by the filter with the equivalent
brick-wall filter bandwidth. The amplifier and filter noise can be calculated using Equation 2.
(2)
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Product Folder Link(s): PGA870