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PROGRAMMABLE POST-GAIN AMPLIFIER PROGRAMMABLE CLAMPING
LOW-PASS FILTER
A1
R
G
A2
Clamp
Gain
Control
Bits
From
Attenuator
Clamp
Control
Bit
To
Low-Pass
Filter
PGA
V
CM
(+1.65V)
OUT
( )
OUT
VCA8500
SBOS390A – JANUARY 2008 – REVISED MARCH 2008
(PGA)
To further optimize the overload recovery behavior of
Following the VCA is a programmable post-gain a complete TGC channel, the VCA8500 integrates a
amplifier (PGA). Figure 62 shows a simplified programmable clamping stage, as shown in
schematic of the PGA, including the clamping stage. Figure 63 . This clamping stage precedes the
The gain of this PGA can be configured to four low-pass filter in order to prevent the filter circuit from
different gain settings: 20dB, 25dB, 27dB, and 30dB, being driven into overload, the result of which would
programmable through the serial port; see Table 8 . be an extended recovery time. Programmable
through the serial interface, the clamping level can be
The PGA structure consists of a differential,
either set to clamp the output to approximately 1.7V
PP
programmable-gain voltage-to-current converter
differential, or be disabled. Disabling the clamp
stage followed by transimpedance amplifiers to create
function increases the current consumption on the
and buffer each side of the differential output. Low
3.3V analog supply (AVDD1) by about 3mA for the
input noise is also a requirement for the PGA design
full device. Note that with the clamp function enabled,
as a result of the large amount of signal attenuation
the third-harmonic distortion increases.
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
As part of a typical data acquisition system, the signal
PGA noise dominates.
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)
Figure 63. Clamping Stage and Low-Pass Filter (Simplified Schematic)
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