Circuit Note

CN-0150

Rev. C | Page 3 of 5

4.0k

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

0

4

3

2

1

0

–1

–2

–3

–4

–70

–60

–50

–40

–30

–20

–10

0

10

INPUT POWER (dBm)

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

Figure 4. Input = 900 MHz, ADC Using an External 2.5 V Reference

4.0k

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

0

4

3

2

1

0

–1

–2

–3

–4

–70

–60

–50

–40

–30

–20

–10

0

10

INPUT POWER (dBm)

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

Figure 5. Input = 1.9 GHz, ADC Using an Internal 2.5 V Reference

4.0k

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

0

4

3

2

1

0

–1

–2

–3

–4

–70

–60

–50

–40

–30

–20

–10

0

10

INPUT POWER (dBm)

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

Figure 6. Input = 1.9 GHz, ADC Using an External 2.5 V Reference

4.0k

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

0

4

3

2

1

0

–1

–2

–3

–4

–70

–60

–50

–40

–30

–20

–10

0

10

INPUT POWER (dBm)

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

Figure 7. Input = 2.2 GHz, ADC Using an Internal 2.5 V Reference

4.0k

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

0

4

3

2

1

0

–1

–2

–3

–4

–70

–60

–50

–40

–30

–20

–10

0

10

INPUT POWER (dBm)

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

Figure 8. Input = 2.2 GHz, ADC Using an External 2.5 V Reference

COMMON VARIATIONS

The AD7887 is a 2-channel, 12-bit ADC with an SPI interface.

The second input channel of this device can be connected to the

AD8318 TEMP pin. This provides a convenient measure of the

ambient temperature around the AD8318. Like the AD8318

power measurement output, the TEMP voltage output should

also be calibrated.

If the end application requires only a single channel, the 12-bit

AD7495 can be used. In multichannel applications that require

multiple ADCs and DAC channels, the AD7294 can be used.

In addition to providing four 12-bit DAC outputs, this subsystem

chip includes four uncommitted ADC channels, two high-side

current sense inputs, and three temperature sensors. Current

and temperature measurements are digitally converted and

The temperature stability of the circuit can be improved using an

external ADC reference. The AD7887 internal 2.5 V reference has

a 50 ppm/°C drift, which is approximately 15 mV over a 125°C

range. Because the detector has a slope of −24 mV/dB, the ADC

reference drift contributes approximately ±0.3 dB to the temperature

drift error budget. The AD8318 temperature drift is approximately

±0.5 dB over a similar temperature range. (This varies with

frequency. See the AD8318 data sheet for more details.)