AD8361
Rev. C | Page 16 of 24
Output Drive Capability and Buffering
The AD8361 is capable of sourcing an output current of
approximately 3 mA. If additional current is required, a simple
buffering circuit can be used as shown in Figure 51. Similar
circuits can be used to increase or decrease the nominal
conversion gain of 7.5 V/V rms (Figure 49 and Figure 50). In
Figure 50, the AD8031 buffers a resistive divider to give a slope
of 3.75 V/V rms. In Figure 49, the op amp’s gain of two increases
the slope to 15 V/V rms. Using other resistor values, the slope
can be changed to an arbitrary value. The AD8031 rail-to-rail
op amp, used in these example, can swing from 50 mV to 4.95 V
on a single 5 V supply and operate at supply voltages down to
2.7 V. If high output current is required (>10 mA), the AD8051,
which also has rail-to- rail capability, can be used down to a
supply voltage of 3 V. It can deliver up to 45 mA of output
current.
100pF
0.01µF
0.01µF
AD8361
VOUT
VPOS
COMM PWDN
5kΩ
5kΩ
5V
15V/V rms
AD8031
01088-C-049
Figure 49. Output Buffering Options, Slope of 15 V/V rms
100pF
0.01µF
0.01µF
AD8361
VOUT
VPOS
COMM PWDN
5V
3.75V/V rms
AD8031
5kΩ
5kΩ
10kΩ
01088-C-050
Figure 50. Output Buffering Options, Slope of 3.75 V/V rms
100pF
0.01µF
0.01µF
AD8361
VOUT
VPOS
COMM PWDN
5V
7.5V/V rms
AD8031
01088-C-051
Figure 51. Output Buffering Options, Slope of 7.5 V/V rms
OUTPUT REFERENCE TEMPERATURE DRIFT
COMPENSATION
The error due to low temperature drift of the AD8361 can be
reduced if the temperature is known. Many systems incorporate
a temperature sensor; the output of the sensor is typically
digitized, facilitating a software correction. Using this
information, only a two-point calibration at ambient is required.
The output voltage of the AD8361 at ambient (25°C) can be
expressed by the equation
)
ΟΣ
IN
OUT
VGAINV
where
GAIN is the conversion gain in V/V rms and V
OS
is the
extrapolated output voltage for an input level of 0 V.
GAIN and
V
OS
(also referred to as intercept and output reference) can be
calculated at ambient using a simple two-point calibration by
measuring the output voltages for two specific input levels.
Calibration at roughly 35 mV rms (−16 dBm) and 250 mV rms
(+1 dBm) is recommended for maximum linear dynamic range.
However, alternative levels and ranges can be chosen to suit the
application.
GAIN and V
OS
are then calculated using the
equations
)
IN1IN2
OUT1OUT2
VV
VV
GAIN
−
=
)
IN1
OUT1
OS
VGAINVV
Both
GAIN and V
OS
drift over temperature. However, the drift
of V
OS
has a bigger influence on the error relative to the output.
This can be seen by inserting data from Figure 18 and Figure 21
(intercept drift and conversion gain) into the equation for V
OUT
.
These plots are consistent with Figure 14 and Figure 15, which
show that the error due to temperature drift decreases with
increasing input level. This results from the offset error having a
diminishing influence with increasing level on the overall
measurement error.
From Figure 18, the average intercept drift is 0.43 mV/°C from
−40°C to +25°C and 0.17 mV/°C from +25°C to +85°C. For a
less rigorous compensation scheme, the average drift over the
complete temperature range can be calculated as
()
()
()
C/V0.000304
C40C85
V0.028V0.010
C/V °=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
°−−°+
−−
=°
VOS
DRIFT
With the drift of
V
OS
included, the equation for V
OUT
becomes
V
OUT
= (GAIN × V
IN
) + V
OS
+ DRIFT
VOS
× (TEMP − 25°C)
