VCA8500_0803 Datasheet, PDF (25/40 Pages) Texas Instruments – 8-Channel, Ultralow-Power, Variable Gain Amplifier with Low-Noise Pre-Amp

VCA8500
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PROGRAMMABLE POST-GAIN AMPLIFIER
(PGA)
Following the VCA is a programmable post-gain
amplifier (PGA). Figure 62 shows a simplified
schematic of the PGA, including the clamping stage.
The gain of this PGA can be configured to four
different gain settings: 20dB, 25dB, 27dB, and 30dB,
programmable through the serial port; see Table 8.
The PGA structure consists of a differential,
programmable-gain voltage-to-current converter
stage followed by transimpedance amplifiers to create
and buffer each side of the differential output. Low
input noise is also a requirement for the PGA design
as a result of the large amount of signal attenuation
that can be applied in the preceding VCA stage. At
minimum VCA attenuation (used for small input
signals), the LNA noise dominates; at maximum VCA
attenuation (large input signals), the attenuator and
PGA noise dominates.
A1
From
Attenuator
Gain
Control
R_G
Bits
Clamp
Control
Bit
Clamp
To
Low-Pass
Filter
A2
SBOS390A – JANUARY 2008 – REVISED MARCH 2008
PROGRAMMABLE CLAMPING
To further optimize the overload recovery behavior of
a complete TGC channel, the VCA8500 integrates a
programmable clamping stage, as shown in
Figure 63. This clamping stage precedes the
low-pass filter in order to prevent the filter circuit from
being driven into overload, the result of which would
be an extended recovery time. Programmable
through the serial interface, the clamping level can be
either set to clamp the output to approximately 1.7V_PP
differential, or be disabled. Disabling the clamp
function increases the current consumption on the
3.3V analog supply (AVDD1) by about 3mA for the
full device. Note that with the clamp function enabled,
the third-harmonic distortion increases.
LOW-PASS FILTER
As part of a typical data acquisition system, the signal
bandwidth generally must be limited by the use of an
anti-aliasing filter before the analog-to-digital
converter (ADC). The VCA8500 integrates such an
anti-aliasing filter in the form of a programmable
low-pass filter (LPF) for each channel. The LPF is
designed as a differential, active, second-order filter
that approximates a Butterworth characteristic, with
typically 12dB per octave roll-off. Figure 63 shows the
simplified schematic of half the differential active
low-pass filter. Programmable through the serial
interface, the –3dB frequency corner can be set to
either 10MHz or 15MHz. The filter is set for all
channels simultaneously.
Figure 62. Post-Gain Amplifier
(Simplified Schematic)
PGA
OUT
(OUT)
V_CM
(+1.65V)
Figure 63. Clamping Stage and Low-Pass Filter (Simplified Schematic)
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): VCA8500
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