AD8361ARMZ datasheet(16/24 Pages) AD

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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

0.01

µF

0.01

µF

AD8361

VOUT

VPOS

COMM PWDN

Ω

15V/V rms

AD8031

Figure 49. Output Buffering Options, Slope of 15 V/V rms

100pF

0.01

µF

0.01

µF

AD8361

VOUT

VPOS

COMM PWDN

3.75V/V rms

AD8031

Ω

10k

Ω

Figure 50. Output Buffering Options, Slope of 3.75 V/V rms

100pF

0.01

µF

0.01

µF

AD8361

VOUT

VPOS

COMM PWDN

7.5V/V rms

AD8031

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

OUT

GAIN

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

(

)

IN1

IN2

OUT1

OUT2

GAIN

−

(

)

IN1

OUT1

GAIN

−

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

()

0.000304

0.028

0.010

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

VOS

DRIFT

With the drift of VOS included, the equation for VOUT becomes

VOUT

= (GAIN × VIN) + VOS + DRIFTVOS × (TEMP − 25°C)

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