VCA2613 Datasheet, PDF (12/16 Pages) Burr-Brown (TI) – Dual, VARIABLE GAIN AMPLIFIER with Low-Noise Preamp

R_S
INPUT
VCM
Q_1A
Q_1B
Q_2A
Q_2B
Q_3A
Q_3B
Q_4A
Q_4B
OUTPUT
Q_5A
Q_5B
A1
A2
A3
A4
A5
B1
B2
PROGRAMMABLE ATTENUATOR SECTION
FIGURE 13. Programmable Attenuator Section.
comparator input threshold, the VCA output is blocked and
an appropriate fixed DC level is substituted, providing fast
and clean overload recovery. The basic architecture is shown
in Figure 14. Both high and low overrange conditions are
sensed and corrected by this circuit.
VCA_CNTL = 3.0V, DIFFERENTIAL, MGS = 100, (36dB)
Output
From VCA
Comparators
Selection
Logic
Output
PGA
Gain = A
–^{E E}
AA
E = Maximum Peak Amplitude
FIGURE 14. Overload Protection Circuitry.
Figures 15 and 16 show typical overload recovery wave-
forms with MGS = 100, for VCA + PGA minimum gain (0dB)
and maximum gain (36dB), respectively. LNP gain is set to
25dB in both cases.
VCA_CNTL = 0.2V, DIFFERENTIAL, MGS = 100, (0dB)
Output
Input
200ns/div
FIGURE 16. Overload Recovery Response For Maximum Gain.
INPUT OVERLOAD RECOVERY
One of the most important applications for the VCA2613 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.
To prevent damage, it is necessary to place a protection
circuit between the transducer and the VCA2613, as shown
in Figure 17. Care must be taken to prevent any signal from
turning the ESD diodes on. Turning on the ESD diodes inside
the VCA2613 could cause the input coupling capacitor (C_C)
to charge to the wrong value.
V_DD
C_F
R_F
Input
Protection
Network
LNP_INP
LNP
LNP_OUTN
200ns/div
FIGURE 15. Overload Recovery Response For Minimum Gain.
ESD Diode
FIGURE 17. VCA2613 Diode Bridge Protection Circuit.
12
VCA2613
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