AD7148

Rev. 0 | Page 20 of 56

PROXIMITY SENSITIVITY

The fast filter in Figure 30 is used to detect when someone is

close to the sensor (proximity). Two conditions set the internal

proximity detection signal, using Comparator 1 and Comparator 2.

Comparator 1 detects when a user is approaching a sensor. The

PROXIMITY_DETECTION_RATE bits (Address0x003[13:8])

controls the sensitivity of Comparator 1. For example, if

PROXIMITY_DETECTION_RATE is set to 4, the Proximity 1

signal is set when the absolute difference between WORD1 and

WORD3 exceeds (4 × 16) LSB codes. Comparator 2 detects when

a user hovers over a sensor or approaches a sensor very slowly.

The PROXIMITY_RECAL_LVL bits (Address 0x003[7:0]) control

the sensitivity of Comparator 2. For example, if PROXIMITY_

RECAL_LVL is set to 75, the Proximity 2 signal is set when the

absolute difference between the fast filter average value and the

ambient value exceeds (75 × 16) LSB codes.

FF_SKIP_CNT

The proximity detection fast FIFO is used by the on-chip logic

to determine if proximity is detected . The fast FIFO expects to

receive samples from the converter at a set rate. FF_SKIP_CNT

(Register 0x002[3:0]) is the fast filter skip control, which used to

normalize the frequency of the samples going into the FIFO,

regardless of how many conversion stages are in a sequence. This

value determines which CDC samples are not used (skipped) in

the proximity detection fast FIFO.

Determining the FF_SKIP_CNT value is required only once during

the initial setup of the capacitance sensor interface. Table 13 shows

how FF_SKIP_CNT controls the update rate to the fast FIFO.

Recommended value for this setting, when using all eight

conversion stages on the AD7148, is

FF_SKIP_CNT = 0000 = no samples skipped.

8

7

N

= 0

PROXIMITY_DETECTION_RATE

REGISTER 0x003

PROXIMITY_RECAL_LVL

REGISTER 0x003

BANK 3 REGISTERS

SLOW_FILTER_UPDATE_LVL

REGISTER 0x003

BANK 3 REGISTERS

SW1

16

NOTES

1. SLOW_FILTER_EN IS SET AND SW1 IS CLOSED WHEN |STAGEx_SF_WORD0–STAGEx_SF_WORD 1| EXCEEDS THE VALUE PROGRAMMED IN THE SLOW_FILTER_UPDATE_LVL BITS

PROVIDING PROXIMITY IS NOT SET.

2. PROXIMITY 1 IS SET WHEN |STAGEx_FF_WORD0– STAGEx_FF_WORD3| EXCEEDS THE VALUE PROGRAMMED IN THE PROXIMITY_DETECTION_RATE BITS.

3. PROXIMITY 2 IS SET WHEN |AVERAGE–AMBIENT| EXCEEDS THE VALUE PROGRAMMED IN THE PROXIMITY_RECAL_LVL BITS.

4. DESCRIPTION OF COMPARATOR FUNCTIONS:

COMPARATOR 1: USED TO DETECT WHEN A USER IS APPROACHING OR LEAVING A SENSOR.

COMPARATOR 2: USED TO DETECT WHEN A USER IS HOVERING OVER A SENSOR OR APPROACHING A SENSOR VERY SLOWLY.

FP_PROXIMITY_RECAL

REGISTER 0x004

LP_PROXIMITY_RECAL

REGISTER 0x004

PROXIMITY

AMBIENT VALUE

STAGEx_SF_WORDx

TIME

SENSOR

CONTACT

STAGEx_SF_AMBIENT

BANK 3 REGISTERS

STAGEx_FF_AVG

BANK 3 REGISTERS

FP_PROXIMITY_CNT

REGISTER 0x002

LP_PROXIMITY_CNT

REGISTER 0x002

PROXIMITY 1

PROXIMITY TIMING

CONTROL LOGIC

WORD(N)

Σ

STAGEx_SF_WORD7

STAGEx_SF_WORD6

STAGEx_SF_WORD5

STAGEx_SF_WORD4

STAGEx_SF_WORD3

STAGEx_SF_WORD2

STAGEx_SF_WORD1

STAGEx_SF_WORD0

STAGEx_FF_WORDx

ALSO USED TO DETECT IF THE SENSOR AMBIENT LEVEL HAS CHANGED AS A RESULT OF USER INTERACTION.

FOR EXAMPLE, HUMIDITY OR DIRT LEFT BEHIND ON SENSOR.

COMPARATOR 3: USED TO ENABLE THE SLOW FILTER UPDATE RATE. THE SLOW FILTER IS UPDATED WHENSLOW_FILTER_EN IS SET AND PROXIMITY IS NOT SET.

WORD0 – WORD3

CDC

STAGEx_FF_WORD0

STAGEx_FF_WORD1

STAGEx_FF_WORD2

STAGEx_FF_WORD3

STAGEx_FF_WORD4

STAGEx_FF_WORD5

STAGEx_FF_WORD6

STAGEx_FF_WORD7

AVERAGE – AMBIENT

PROXIMITY

COMPARATOR 1

COMPARATOR 2

COMPARATOR 3

SLOW_FILTER_EN

WORD0 – CDC VALUE

Figure 30. Proximity Detection Logic