AD7148 Analog Devices, AD7148 Datasheet - Page 20

• Manufacturer Part Number: AD7148
• Manufacturer: Analog Devices

Specifications of AD7148

• Resolution (bits): 16bit
• # Chan: 8
• Sample Rate: 250kSPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: SE-Uni
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD7148
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.
SLOW_FILTER_EN
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
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:
PROVIDING PROXIMITY IS NOT SET.
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.
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.
SLOW_FILTER_UPDATE_LVL
PROXIMITY
CDC
WORD0 – CDC VALUE
REGISTER 0x003
COMPARATOR 3
16
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.
STAGEx_FF_WORD0
STAGEx_FF_WORD1
STAGEx_FF_WORD2
STAGEx_FF_WORD3
STAGEx_FF_WORD4
STAGEx_FF_WORD5
STAGEx_FF_WORD6
STAGEx_FF_WORD7
Σ
N = 0
7
WORD(N)
STAGEx_SF_WORD0
STAGEx_SF_WORD1
STAGEx_SF_WORD2
STAGEx_SF_WORD3
STAGEx_SF_WORD4
STAGEx_SF_WORD5
STAGEx_SF_WORD6
STAGEx_SF_WORD7
BANK 3 REGISTERS
8
BANK 3 REGISTERS
SW1
BANK 3 REGISTERS
PROXIMITY_DETECTION_RATE
STAGEx_FF_AVG
WORD0 – WORD3
REGISTER 0x003
COMPARATOR 1
STAGEx_SF_AMBIENT
BANK 3 REGISTERS
Figure 30. Proximity Detection Logic
Rev. A | Page 20 of 56
PROXIMITY_RECAL_LVL
AVERAGE – AMBIENT
COMPARATOR 2
REGISTER 0x003
PROXIMITY 1
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.
FP_PROXIMITY_RECAL
FP_PROXIMITY_CNT
PROXIMITY
REGISTER 0x002
REGISTER 0x004
STAGEx_SF_WORDx
STAGEx_FF_WORDx
PROXIMITY TIMING
CONTROL LOGIC
CONTACT
SENSOR
LP_PROXIMITY_RECAL
LP_PROXIMITY_CNT
REGISTER 0x002
REGISTER 0x004
AMBIENT VALUE
TIME