AD8361_1 Datasheet, PDF (16/24 Pages) Analog Devices – LF to 2.5 GHz TruPwr™ Detector

AD8361
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.
5V
0.01µF 100pF
VPOS
VOUT
AD8361
0.01µF
AD8031
15V/V rms
COMM PWDN
5kΩ
5kΩ
Figure 49. Output Buffering Options, Slope of 15 V/V rms
0.01µF 100pF
VPOS
VOUT
5kΩ
AD8361
5kΩ
COMM PWDN
5V
10kΩ
0.01µF
AD8031
3.75V/V rms
Figure 50. Output Buffering Options, Slope of 3.75 V/V rms
0.01µF 100pF
VPOS
VOUT
AD8361
COMM PWDN
5V
0.01µF
AD8031
7.5V/V rms
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
V_OUT = (GAIN ×V_IN ) + ς_ΟΣ
where GAIN is the conversion gain in V/V rms and VOS is the
extrapolated output voltage for an input level of 0 V. GAIN and
VOS (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 VOS are then calculated using the
equations
( ) _{GAIN =}
V_OUT2 − V_OUT1
V_IN2 − V_IN1
V_OS = V_OUT1 − (GAIN ×V_IN1 )
Both GAIN and VOS drift over temperature. However, the drift
of VOS 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 VOUT.
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
( ) ^{( )} DRIFT_VOS
V/°C
=
⎜⎜⎝⎛
0.010 V −
+ 85°C −
− 0.028 V
(− 40°C)
⎟⎟⎠⎞ ^{= 0.000304 V/°C}
With the drift of VOS included, the equation for VOUT becomes
VOUT = (GAIN × VIN) + VOS + DRIFTVOS × (TEMP − 25°C)
Rev. C | Page 16 of 24