ADAF1080BCPZ datasheet(27/36 Pages) AD

27 / 36 page

Data Sheet

ADAF1080

APPLICATIONS INFORMATION

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Rev. 0 | 27 of 36

Figure 54. Synchronization of the ADAF1080 and the ADC Sampling for Slow

Sampling Rate

Applications that do not require precision performance at low fre-

quencies can disable the chopping functionality of the integrated

PGIA by tying the SYNC pin to VDD. When the chopping feature

of the PGIA is disabled, the full bandwidth of the PGIA can be

used for settling; however, higher flicker noise at lower frequencies

is observed. Refer to Figure 37 and Figure 36 for PGIA noise

performance with and without chopping.

Table 15 outlines the different modes available for the integrated

amplifier.

Table 15. SYNC Pin Functionality

SYNC Pin State

SYNC_EN Pin

Instrumentation Amplifier

Chopping Functionality

VDD

Don’t care

Chopping disabled

GND

Don’t care

Internal 200 kHz chopping

400 kHz < fSYNC <

1.6 MHz

Low

Internal 200 kHz chopping

400 kHz < fSYNC <

1.6 MHz

High

fSYNC/4 chopping

If the SYNC and SYNC_EN pins are left floating, two internal

300 kΩ pull-downs ensure that the amplifier is chopping at 200 kHz

as a default condition.

ELECTRICAL OFFSET CANCELLATION

The integrated flip coil driver can be used to reverse the sensitivity

polarity so that all error sources related to the electrical offset and

offset drift of the AMR sensor and signal conditioning circuit can be

calibrated out of the sense magnetic field measurement.

For one time offset cancellation, in the presence of constant or low

frequency magnetic fields, two measurements can be made with

reverse states of the flip coil. The offset can then be calculated

directly by the following equation:

VOFFSET=VOUTFn+VOUTFp

−VSET2

(8)

where:

VOUTFn is the output of the ADAF1080 when FLIP_DRV is GND.

VOUTFp is the output of theADAF1080 when FLIP_DRV is VDD.

VSET is the voltage on the VSET pin.

Once the electrical offset is measured, this offset can be canceled

in any further measurements as shown in the following equation:

VOUT_OC=VOUT−VSET2−VOFFSET

(9)

where VOUT_OC is the offset canceled output voltage of the

ADAF1080.

For periodic offset cancellation, the flip coil can be reversed at a

fixed frequency, and the offset and offset drift can be removed by

using the following equation:

VOUT_OC=VOUTFn−VOUTFp

(10)

When using the flip coil driver, sampling of the output must take

place after the settling time of the sensor (tSETTLING) is complete, as

shown in the Flip Coil and Measurement Timing section.

EXAMPLE OFFSET CALCULATION

The example offset calculation is an example where a 1 mT

external field (BEXT) is applied along the sense field axis of the

ADAF1080 as shown in Figure 47.

In this example, the following is assumed:

►

A0 and A1 = VDD, G = 80

►

VDD = VSET = 5 V and SCOEFF1 = 199.25 mV/mT based on the

typical value for the ±2 mT magnetic range

►

The electrical offset of the ADAF1080 and its signal chain is

2 mV

In this setup, the expected output of the ADAF1080 is the following

when FLIP_DRV = GND:

VOUTFn=

‐199.25 mVmT×1 mT+5 V2+2 mV=2.303 V (11)

In this setup, the expected output of the ADAF1080 is the following

when FLIP_DRV = VDD:

VOUTFp=

199.25 mVmT×1 mT+5 V2+2 mV=2.701 V (12)

By combining these two measurements, the electrical offset can

then be calculated as follows:

VOFFSET=

VOUTFn+VOUTFp

−VSET2=2.502 V−5 V2=2 mV (13)

Once the electrical offset is calculated, the result can be used for all

future measurement as shown as follows to remove electrical offset

errors from the output magnetic sense field measurements:

VOUT_OC=VOUT−VSET2−VOFFSET

(14)

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