The capacitance that is determined in Equation 5 should be
added to the capacitance of Equation 4 to determine the
overall bandwidth of the LNP. The LNPINNA (pin 12) and the
LNPINNB (pin 25) should be bypassed to ground by the
shortest means possible to avoid any inductance in the lead.
The attenuator is comprised of two sections, with five parallel
clipping amplifier/FET combinations in each. Special refer-
ence circuitry is provided so that the (VCM – VT) limit voltage
will track temperature and IC process variations, minimizing
the effects on the attenuator control characteristic.
In addition to the analog VCACNTL gain setting input, the
attenuator architecture provides digitally programmable ad-
justment in eight steps, via the three MGS bits. These adjust
the maximum achievable gain (corresponding to minimum
attenuation in the VCA, with VCACNTL = 3.0V) in 3dB incre-
ments. This function is accomplished by providing multiple
FET sub-elements for each of the Q1 to Q10 FET shunt
elements (see Figure 12). In the simplified diagram of
Figure 13, each shunt FET is shown as two sub-elements,
QNA and QNB. Selector switches, driven by the MGS bits,
activate either or both of the sub-element FETs to adjust the
maximum RON and thus achieve the stepped attenuation
options.
LNP OUTPUT BUFFER
The differential LNP output is buffered by wideband class AB
voltage followers which are designed to drive low impedance
loads. This is necessary to maintain LNP gain accuracy,
since the VCA input exhibits gain-dependent input imped-
ance. The buffers are also useful when the LNP output is
brought out to drive external filters or other signal processing
circuitry. Good distortion performance is maintained with
buffer loads as low as 135Ω. As mentioned previously, the
buffer inputs are AC-coupled to the LNP outputs with a
3.6kHz high-pass characteristic, and the DC common-mode
level is maintained at the correct VCM for compatibility with
the VCA input.
The VCA can be used to process either differential or single-
ended signals. Fully differential operation will reduce 2nd-
harmonic distortion by about 10dB for full-scale signals.
VOLTAGE-CONTROLLED ATTENUATOR (VCA)—DETAIL
Input impedance of the VCA will vary with gain setting, due
to the changing resistances of the programmable voltage
divider structure. At large attenuation factors (that is, low gain
settings), the impedance will approach the series resistor
value of approximately 135Ω.
The VCA is designed to have a dB-linear attenuation charac-
teristic; that is, the gain loss in dB is constant for each equal
increment of the VCACNTL control voltage. See Figure 1 for a
block diagram of the VCA. The attenuator is essentially a
variable voltage divider consisting of one series input resis-
tor, RS, and ten identical shunt FETs, placed in parallel and
controlled by sequentially activated clipping amplifiers. Each
clipping amplifier can be thought of as a specialized voltage
comparator with a soft transfer characteristic and well-con-
trolled output limit voltages. The reference voltages V1 through
V10 are equally spaced over the 0V to 3.0V control voltage
range. As the control voltage rises through the input range of
each clipping amplifier, the amplifier output will rise from 0V
(FET completely ON) to VCM – VT (FET nearly OFF), where
VCM is the common source voltage and VT is the threshold
voltage of the FET. As each FET approaches its OFF state
and the control voltage continues to rise, the next clipping
amplifier/FET combination takes over for the next portion of
the piecewise-linear attenuation characteristic. Thus, low
control voltages have most of the FETs turned ON, while
high control voltages have most turned OFF. Each FET acts
to decrease the shunt resistance of the voltage divider
formed by RS and the parallel FET network.
As with the LNP stage, the VCA output is AC-coupled into the
PGA. This means that the attenuation-dependent DC com-
mon-mode voltage will not propagate into the PGA, and so
the PGA’s DC output level will remain constant.
Finally, note that the VCACNTL input consists of FET gate
inputs. This provides very high impedance and ensures that
multiple VCA2616 and VCA2611 devices may be connected
in parallel with no significant loading effects. The nominal
voltage range for the VCACNTL input spans from 0V to 3V.
Overdriving this input (≤ 5V) does not affect the performance.
INPUT OVERLOAD RECOVERY
One of the most important applications for the VCA2616 and
VCA2611 is processing signals in an ultrasound system. The
ultrasound signal flow begins when a large signal is applied to
a transducer, which converts electrical energy to acoustic
energy. It is not uncommon for the amplitude of the electrical
signal that is applied to the transducer to be ±50V or greater.
RS
OUTPUT
INPUT
Q1A
Q1B
Q2A
Q2B
Q3A
Q3B
Q4A
Q4B
Q5A
Q5B
VCM
A1
A2
A3
A4
A5
B1
B2
Programmable Attenuator Section
FIGURE 13. Programmable Attenuator Section.
14
VCA2616, VCA2611
SBOS234E
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