(= maximum attenuation). For VCACNTL = 3V (no attenuation),
the VCA + PGA gain will be controlled by the programmed
PGA gain (24 to 45 dB in 3dB steps). For clarity, the gain and
attenuation factors are detailed in Table III.
VDD
CF
RF
MGS
ATTENUATOR GAIN DIFFERENTIAL ATTENUATOR +
SETTING VCACNTL = 0V to 3V
PGA GAIN
DIFF. PGA GAIN
LNPOUTN
000
001
010
011
100
101
110
111
–24dB to 0dB
–27dB to 0dB
–30dB to 0dB
–33dB to 0dB
–36dB to 0dB
–39dB to 0dB
–42dB to 0dB
–45dB to 0dB
24dB
27dB
30dB
33dB
36dB
39dB
42dB
45dB
0dB to 24dB
0dB to 27dB
0dB to 30dB
0dB to 33dB
0dB to 36dB
0dB to 39dB
0dB to 42dB
0dB to 45dB
LNPIN
P
Protection
Network
LNP
ESD Diode
FIGURE 14. VCA2616 and VCA2611 Diode Bridge Protection
Circuit.
TABLE III. MGS Settings.
To prevent damage, it is necessary to place a protection circuit
between the transducer and the VCA2616 and VCA2611 (see
Figure 14). Care must be taken to prevent any signal from
turning the ESD diodes on. Turning on the ESD diodes inside
the VCA2616 and VCA2611 could cause the input coupling
capacitor (CC) to charge to the wrong value.
The PGA architecture consists of a differential, program-
mable-gain voltage to current converter stage followed by
transimpedance amplifiers to create and buffer each side of
the differential output. The circuitry associated with the volt-
age to current converter is similar to that previously de-
scribed for the LNP, with the addition of eight selectable PGA
gain-setting resistor combinations (controlled by the MGS
bits) in place of the fixed resistor network used in the LNP.
Low input noise is also a requirement of the PGA design due
to the large amount of signal attenuation which can be
inserted between the LNP and the PGA. At minimum VCA
attenuation (used for small input signals) the LNP noise
dominates; at maximum VCA attenuation (large input sig-
nals) the PGA noise dominates. Note that if the PGA output
is used single-ended, the apparent gain will be 6dB lower.
PGA POST-AMPLIFIER—DETAIL
Figure 15 shows a simplified circuit diagram of the PGA block.
As described previously, the PGA gain is programmed with
the same MGS bits which control the VCA maximum attenu-
ation factor. Specifically, the PGA gain at each MGS setting is
the inverse (reciprocal) of the maximum VCA attenuation at
that setting. Therefore, the VCA + PGA overall gain will always
be 0dB (unity) when the analog VCACNTL input is set to 0V
VDD
To Bias
Circuitry
Q1
Q11
Q12
Q9
RL
RL
VCAOUT
P
VCAOUTN
Q3
Q8
VCM
VCM
RS1
RS2
Q13
Q4
Q7
+In
–In
Q14
Q2
Q10
Q5
Q6
To Bias
Circuitry
FIGURE 15. Simplified Block Diagram of the PGA Section Within the VCA2616 and VCA2611.
VCA2616, VCA2611
16
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