Preliminary Technical Data

AD5801

Rev. 0 | Page 11 of 13

A programmable safety interval is allowed to elapse before the

actual position measurement is made by the ADC. This safety

interval duration can be set in a register to be anything from

zero to 1000µs. Four consecutive measurements from the lens

position signal are made and their average saved to result

registers. Each of the lens position measurement results are

stored in two 8-bit registers because the ADC is a twelve-bit

converter. It is possible to turn-off the averaging feature if

required.

The ADC has the ability to accept either current or voltage

inputs depending on the position sensing scheme used. A bit in

the CONFIG register will set the AD5801 for current or voltage

conversion. Figure 6 show a simplified diagram where the ADC

measures the output of an optical reflective position sensor.

Depending on whether it is the auto focus lens position or

Zoom position you are measuring the IDAC sources a current

derived from an integrated Bias circuit and external precision

resistor, BIASRES. The Bias circuit consists of bandgap voltage

reference and current to voltage generator.

The current sourced is between 4mA and 19mA with 4-bit

resolution. In the case of an auto focus lens position

measurement the desired current is programmed to LED, D1.

The incident light from D1 falls on the photosensitive device

Q1 . The output of Q1 is connected to the POSSENSE1 pin and

the current flowing in Q1 is then measured by the ADC, and

there is a direct current to lens position relationship which

indicates the position of the auto focus lens. Because the zoom

lens position LED, D2, has no current flowing in it then there is

no current flowing in the photosensitive device Q2, and only

the position of the auto focus lens is detected and measured.

The position sensing for the zoom lens works on exactly the

same principles.

OPTICAL

REFLECTIVE

POSITION

FEEDBACK

SCHEME

AD5801

Figure 6. Lens position Sensing using an Optical Reflective

Feedback Scheme.

Figure 7 shows the ADC configured to measure in voltage

mode. The inputs POSSENSE1 and POSSENSE2 can be

connected to the outputs of HALL sensors, or POSSENE1 can

be tied to AGND and POSSENSE2 can be used to measure

single ended voltages, The AD5801 also has an internal register

which may be programmed with a required threshold value,

and this value can then be compared to the Hall voltage or

single-ended voltage input. The threshold voltage can be used

to indicate the ret position of either lens. For example, if the

user is driving the lens to the start position the output of the

comparator disables the output drivers when the programmed

threshold indicating the lens start position is reached. Only one

position sense measurement is performed at a time. For a zoom

lens position measurement current is source from the

POSSENSEZ pin into the Zoom Hall Plate, and the resulting

voltage from the output of the Hall sensor is connected through

the POSSENSE1 and POSSENSE2 pins to the ADC. Given that

only one Hall Sensor is active at one time it is possible to

connect the outputs of both sensors together. For single-ended

sensor outputs the output voltage is connected to POSSENSE2

and POSSENSE1 is connected to AGND.