AD8362
CIRCUIT DESCRIPTION
The AD8362 is a fully calibrated, high accuracy, rms-to-dc
converter providing a measurement range of over 65 dB. It is
capable of operating from signals as low in frequency as a few
hertz to at least 3.8 GHz. Unlike earlier rms-to-dc converters,
the response bandwidth is completely independent of the
signal magnitude. The −3 dB point occurs at about 3.5 GHz.
The capacity of this part to accurately measure waveforms
having a high peak-to-rms ratio (crest factor) is independent
of either the signal frequency or its absolute magnitude, over
a wide range of conditions.
The VGA gain has the form
G
SET = GO exp(−VSET/VGNS
)
(2)
(3)
where:
GO is a basic fixed gain.
V
GNS is a scaling voltage that defines the gain slope (the dB
change per volt). Note that the gain decreases with VSET
.
The VGA output is
V
SIG = GSETVIN = GOVIN exp(VSET/VGNS)
where VIN is the ac voltage applied to the input terminals
of the AD8362.
This unique combination allows the AD8362 to be used as a
calibrated RF wattmeter covering a power ratio of >1,000,000:1,
a power controller in closed-loop systems, a general-purpose
rms-responding voltmeter, and in many other low frequency
applications.
As explained in the Recommended Input Coupling section, the
input drive can either be single-sided or differential, although
dynamic range is maximized with a differential input drive. The
effect of high frequency imbalances when using a single-sided
drive is less apparent at low frequencies (from 50 Hz to 500 MHz),
but the peak input voltage capacity is always halved relative to
differential operation.
The part comprises the core elements of a high performance
AGC loop (see Figure 44), laser-trimmed during manufacturing
to close tolerances while fully operational at a test frequency of
100 MHz. Its linear, wideband VGA provides a general voltage
gain, GSET; this can be controlled in a precisely exponential (linear-
in-dB) manner over the full 68 dB range from −25 dB to +43 dB
by a voltage, VSET. However, to provide adequate guardbanding,
only the central 60 dB of this range, from −21 dB to +39 dB, is
normally used. The Adjusting VTGT to Accommodate Signals
with Very High Crest Factors section shows how this basic
range can be shifted up or down.
SQUARE LAW DETECTION
The output of the variable gain amplifier (VSIG) is applied to
a wideband square law detector, which provides a true rms
response to this alternating signal that is essentially independent
of waveform. Its output is a fluctuating current (ISQU) that has
a positive mean value. This current is integrated by an on-chip
capacitance (CF), which is usually augmented by an external
capacitance (CLPF) to extend the averaging time. The resulting
voltage is buffered by a gain of 5, dc-coupled amplifier whose
rail-to-rail output (VOUT) can be used for either measurement
or control purposes.
AMPLITUDE TARGET
FOR V
SIG
–25dB TO +43dB
INHI
MATCH WIDE-
BAND SQUARERS
VTGT
VGA
2
2
X
× 0.06
X
V
V
ATG
In most applications, the AGC loop is closed via the setpoint
interface pin, VSET, to which the VGA gain control voltage on
VOUT is applied. In measurement modes, the closure is direct
and local by a simple connection from the output of the VOUT
pin to the VSET pin. In controller modes, the feedback path is
around some larger system, but the operation is the same.
SIG
I
ACOM
INLO
I
SQU
TGT
G
SET
CHPF
OFFSET
NULLING
C
F
OUTPUT
FILTER
VOUT
SETPOINT
INTERFACE
VSET
VREF
The fluctuating current (ISQU) is balanced against a fixed
setpoint target current (ITGT) using current mode subtraction.
With the exact integration provided by the capacitor(s), the
AGC loop equilibrates when
CLPF
INTERNAL
RESISTORS
SET BUFFER
GAIN TO 5
BAND GAP
REFERENCE
MEAN(ISQU) = ITGT
(4)
1.25V
C
LPF
EXTERNAL
The current, ITGT, is provided by a second-reference squaring
cell whose input is the amplitude-target voltage VATG. This is
a fraction of the voltage VTGT applied to a special interface,
which accepts this input at the VTGT pin. Because the two
ACOM
Figure 44. Basic Structure of the AD8362
squaring cells are electrically identical and are carefully imple-
mented in the IC, process and temperature-dependent variations
in the detailed behavior of the two square-law functions cancel.
Accordingly, VTGT (and its fractional part VATG) determines
the output that must be provided by the VGA for the AGC
Rev. D | Page 16 of 32
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