AD8362
In such cases, the input coupling capacitors should be large
VOLTAGE VS. POWER CALIBRATION
enough so that the lowest frequency components of the signal
that are to be included in the measurement are minimally
attenuated. For example, for a 3 dB reduction at 1.5 kHz,
capacitances of 1 µF are needed because the input resistance is
100 Ω at each input pin (200 Ω differentially) and we calculate
1/(2π × 1.5 kΩ × 100) = 1 µF. Also, to lower the high-pass
corner frequency of the VGA, a capacitor of value 200 µF-Hz
should be used between the CHPF pin and ground; to provide a
similar 1.5 kHz high-pass corner, a capacitor of 133 nF should
be used.
The AD8362 can be used as an accurate rms voltmeter
from arbitrarily low frequencies to microwave frequencies.
For low frequency operation, the input is usually specified
either in volts rms or in dBV (decibels relative to 1 V rms).
Driven differentially, the specified input range in dBV runs
from −60 dBV to 0 dBV (1 mV to 1 V rms). In these terms,
the intercept is at −70 dBV.
At high frequencies, signal levels are commonly specified
in power terms. In these circumstances, the source and
termination impedances are an essential part of the overall
scaling. To set the AD8362’s input impedance to 50 Ω, it is
necessary to add a resistor of 66.7 Ω across the internal 200 Ω
differential input impedance of the IC. (This is discussed
further in later sections.) For this condition, the intercept
occurs at a nominal power level of −57 dBm, and VOUT
can be stated in this way:
TIME-DOMAIN RESPONSE OF THE CLOSED LOOP
The external low-pass averaging capacitance, CLPF, added at
the output of the squaring cell, is chosen to provide adequate
filtering of the fluctuating detected signal. The optimum value
depends on the application; as a guideline, a value of roughly
900 µF-Hz should be used. For example, a capacitance of 5 µF
provides adequate filtering down to 180 Hz. Note that the
fluctuation in the quasi-dc output of a squaring cell operating
on a sine wave input is a raised cosine at twice the signal
frequency, easing this filtering function.
VOUT =
PIN + 57
× 50 mV dB
(10)
where PIN is expressed in dBm. For example, an input of
−30 dBm generates an output of 1.35 V.
In the standard connections for the measurement mode,
the VSET pin is tied to VOUT. For small changes in input
amplitude (a few decibels), the time-domain response of this
loop is essentially linear, with a 3 dB low-pass corner frequency
of nominally fLP = 1/(CLPF × 1.1 kΩ). Internal time delays
around this local loop set the minimum recommended value of
this capacitor to about 300 pF, giving fLP = 3 MHz.
EFFECT OF SIGNAL WAVEFORM
The measurement accuracy of an rms-responding device is
ideally unaffected by the waveform of the input signal. This is a
valuable asset in wideband CDMA systems and in many other
modulation modes where there is a significant amount of
random variation of the RF carrier amplitude at baseband
frequencies. The high accuracy of the AD8362 in such cases is
indicated by the Typical Performance Characteristics graphs
and in the Specifications table. Note that at low frequencies, it is
customary to provide a specification of measurement errors due
to waveform effects as a function of the crest factor (σ) rather
than in terms of a system-specific modulation mode.
When large and abrupt changes of input amplitude occur,
the loop response becomes nonlinear and exhibits slew rate
limitations. Further, due to the fundamentals of a system using
transconductance squaring cells as employed in the AD8362,
the slewing is asymmetric for increasing and decreasing inputs.
Figure 44 shows typical waveforms for VOUT for three values
of VIN using CLPF = 1 nF.
When measuring signals whose waveforms have high but
brief peak values (that is, having high crest factors), these
peaks may be clipped, causing a reduction in the apparent value
of the input being measured. This issue is discussed further in
connection with the detailed description of the input system.
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
OPERATION AT LOW FREQUENCIES
In conventional rms-to-dc converters based on junction
techniques, the effective signal bandwidth is proportional to the
signal amplitude. For a 1 MHz rms-to-dc converter, this is the
full-scale bandwidth. However, at an input 60 dB below full-
scale, the bandwidth could be as low as 1 kHz. In sharp contrast,
the 3.5 GHz bandwidth of the VGA in the AD8362 is
independent of its gain. Since this amplifier is internally dc-
coupled, the system can also be used as a high accuracy rms
voltmeter at low frequencies, retaining its temperature-stable
decibel-scaled output, for example, in seismic, audio, and sonar
instrumentation.
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
8
16 24 32 40 48 56 64 72 80 88 96
0
TIME (µs)
Figure 44. Typical Large-Scale Response
Rev. B | Page 17 of 36
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