AD8362
limited to approximately 90 dB. This can only be achieved when
a band-pass filter is used at the operating frequency between
the AD8330 and AD8362.
RMS VOLTMETER WITH 90 dB DYNAMIC RANGE
The 65 dB range of the AD8362 can be extended by adding a
standalone VGA as a preamplifier whose gain control input is
derived directly from VOUT. This extends the dynamic range
by the gain control range of this second amplifier. When this
VGA also provides a linear-in-dB (exponential) gain control
function, the overall measurement remains linearly scaled in
decibels. The VGA gain must decrease with an increase in its
gain bias in the same way as the AD8362. Alternatively, an
inverting op amp with suitable level shifting can be used. It is
convenient to select a VGA needing only a single 5 V supply
and capable of generating a fully balanced differential output.
All of these conditions are met by the AD8330. Figure 66 shows
the schematic. Also, note that the AD8131 is used to convert a
single-ended input into the differential-ended input needed by
the AD8330. The AD8131’s gain of 2 does create a dc offset on
the output of the AD8362, but this is removed by connecting
0.5 V to the VMAG on AD8330.
Figure 65 shows data results of the extended dynamic range at
70 MHz with error in VOUT.
INPUT (dBV)
–103 –93 –83 –73 –63 –53 –43 –33 –23 –13 –3
3.0
7
6
2.5
2.0
1.5
1.0
0.5
0
4
2
0
–2
–4
–6
Using the inverse gain mode (MODE pin low) of the AD8330,
its gain decreases on a slope of 30 mV/dB to a minimum value
of 3 dB for a gain voltage (VDBS) of 1.5 V. VDBS is 40% of the
output of the AD8362. Over the 3 V range from 0.5 V to 3.5 V,
the gain of the AD8330 varies by (0.4 × 3 V)/(30 mV/dB), or
40 dB. Combined with the 65 dB gain span of the AD8362, this
results in a 100 dB variation for a 3 V change in VOUT. Due to
the noise generated from the AD8330, the dynamic range is
–90 –80 –70 –60 –50 –40 –30 –20 –10
0
10
20
INPUT (dBm)
Figure 65. Output and Conformance for the AD8330/AD8362
Extended Dynamic Range Circuit
+5V
0.1µF
GAIN OF 2
AD8362
0.1µF
VPOS
0.1µF
CNTR
0.1µF
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
COMM
CHPF
DECL
INHI
ACOM
VREF
VTGT
VPOS
VOUT
VSET
ACOM
CLPF
10µF
0.1µF
0.01µF
0.1µF
ENBL
VPS1
OFST
VPSO
BAND-PASS
@ 70MHz
AD8131
3
6
8
2
1
4
5
INPUT
49.9Ω
INHI
OPHI
OPLO
CMOP
AD8330
INLO
V
INLO
OUT
0.1µF
29.9Ω
MODE
VDBS
DECL
PWDN
COMM
0.1µF
CMGN
COMM VMAG
0.1µF
–5V
0.01µF
10µF
2kΩ
+0.5V
2kΩ
Figure 66. RMS Voltmeter with 90 dB Dynamic Range
Rev. D | Page 27 of 32
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