AD640 Analog Devices, AD640 Datasheet - Page 10

• Manufacturer Part Number: AD640
• Description: DC-Coupled Demodulating 120 MHz Logarithmic Amplifier
• Manufacturer: Analog Devices

AD640
Input
Waveform
Square Wave
Sine Wave
Sine Wave
Triwave
Triwave
Gaussian Noise
Logarithmic Conformance and Waveform
The waveform also affects the ripple, or periodic deviation from
an ideal logarithmic response. The ripple is greatest for dc or
square wave inputs because every value of the input voltage
maps to a single location on the transfer function and thus
traces out the full nonlinearities in the logarithmic response.
By contrast, a general time varying signal has a continuum of
values within each cycle of its waveform. The averaged output is
thereby “smoothed” because the periodic deviations away from
the ideal response, as the waveform “sweeps over” the transfer
function, tend to cancel. This smoothing effect is greatest for a
triwave input, as demonstrated in Figure 22.
Figure 22. Deviation from Exact Logarithmic Transfer
Function for Two Cascaded AD640s, Showing Effect of
Waveform on Calibration and Linearity
Figure 23. Deviation from Exact Logarithmic Transfer
Function for a Single AD640; Compare Low Level
Response with that of Figure 22
–10
–10
–12
–2
–4
–6
–8
–2
–4
–6
–8
2
0
–80
4
2
0
–70
–70
INPUT AMPLITUDE IN dB ABOVE 1V, AT 10kHz
INPUT AMPLITUDE IN dB ABOVE 1V, AT 10kHz
–60
Peak
or RMS
Either
Peak
rms
Peak
rms
rms
–60
SQUARE WAVE INPUT
–50
SINE WAVE INPUT
Table I.
TRIWAVE INPUT
SQUARE WAVE INPUT
–50
2.718 (e)
Intercept
Factor
1
2
1.414( 2)
1.569(e/ 3)
1.887
SINE WAVE INPUT
TRIWAVE INPUT
–40
–40
–30
–30
Error (Relative
to a DC Input)
0.00 dB
–6.02 dB
–3.01 dB
–8.68 dB
–3.91 dB
–5.52 dB
–20
–20
–10
–10
–10–
The accuracy at low signal inputs is also waveform dependent.
The detectors are not perfect absolute value circuits, having a
sharp “corner” near zero; in fact they become parabolic at low
levels and behave as if there were a dead zone. Consequently,
the output tends to be higher than ideal. When there are enough
stages in the system, as when two AD640s are connected in
cascade, most detectors will be adequately loaded due to the
high overall gain, but a single AD640 does not have sufficient
gain to maintain high accuracy for low level sine wave or triwave
inputs. Figure 23 shows the absolute deviation from calibration
for the same three waveforms for a single AD640. For inputs
between –10 dBV and –40 dBV the vertical displacement of the
traces for the various waveforms remains in agreement with the
predicted dependence, but significant calibration errors arise at
low signal levels.
SIGNAL MAGNITUDE
AD640 is a calibrated device. It is, therefore, important to be
clear in specifying the signal magnitude under all waveform
conditions. For dc or square wave inputs there is, of course, no
ambiguity. Bounded periodic signals, such as sinusoids and
triwaves, can be specified in terms of their simple amplitude
(peak value) or alternatively by their rms value (which is a mea-
sure of power when the impedance is specified). It is generally bet-
ter to define this type of signal in terms of its amplitude because
the AD640 response is a consequence of the input voltage, not
power. However, provided that the appropriate value of inter-
cept for a specific waveform is observed, rms measures may be
used. Random waveforms can only be specified in terms of rms
value because their peak value may be unbounded, as is the case
for Gaussian noise. These must be treated on a case-by-case
basis. The effective intercept given in Table I should be used for
Gaussian noise inputs.
On the other hand, for bounded signals the amplitude can be
expressed either in volts or dBV (decibels relative to 1 V). For
example, a sine wave or triwave of 1 mV amplitude can also be
defined as an input of –60 dBV, one of 100 mV amplitude as
–20 dBV, and so on. RMS value is usually expressed in dBm
(decibels above 1 mW) for a specified impedance level. Through-
out this data sheet we assume a 50
impedance level for high speed systems, when referring to signal power
in dBm. Bearing in mind the above discussion of the effect of
waveform on the intercept calibration of the AD640, it will be
apparent that a sine wave at a power of, say, –10 dBm will not
produce the same output as a triwave or square wave of the
same power. Thus, a sine wave at a power level of –10 dBm has
an rms value of 70.7 mV or an amplitude of 100 mV (that is, 2
times as large, the ratio of amplitude to rms value for a sine
wave), while a triwave of the same power has an amplitude
which is 3 or 1.73 times its rms value, or 122.5 mV.
“Intercept” and “Logarithmic Offset”
If the signals are expressed in dBV, we can write the output in a
simpler form, as
where Input
and X
waveform) in dBV. This form shows more clearly why the intercept
is often referred to as the logarithmic offset. For dc or square
wave inputs, V
and Equation (4) becomes
I
OUT
dBV
= 50 A (Input
is the appropriate value of the intercept (for a given
dBV
X
is the input voltage amplitude (not rms) in dBV
is 1 mV so the numerical value of X
dBV
– X
dBV
)
environment, the customary
Equation (4)
dBV
is –60,
REV. C