Data Sheet AD5246
Rev. C | Page 13 of 16
OPERATION
The AD5246 is a 128-position, digitally controlled variable
resistor (VR) device.
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between Terminal A
and Terminal B is available in 5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ.
The final two or three digits of the part number determine
the nominal resistance value, that is, 10 kΩ = 10, 50 kΩ = 50.
The nominal resistance (R
AB
) of the VR has 128 contact points
accessed by the wiper terminal. The 7-bit data in the RDAC
latch is decoded to select one of the 128 possible settings.
The general equation determining the digitally programmed
output resistance between W and B is
WABWB
RR
D
DR ×+×= 2
128
)(
(1)
where:
D is the decimal equivalent of the binary code loaded in the
7-bit RDAC register.
R
AB
is the end-to-end resistance.
R
W
is the wiper resistance contributed by the on resistance
of each internal switch.
Bx
Wx
Ax
D6
D4
D5
D2
D3
D1
D0
RDAC
LATCH
AND
DECODER
R
S
R
S
R
S
03875-015
Figure 29. AD5246 Equivalent RDAC Circuit
Note that in the zero-scale condition, there is a relatively small
finite wiper resistance. Care should be taken to limit the current
flow between W and B in this state to a maximum pulse current
of no more than 20 mA. Otherwise, degradation or possible
destruction of the internal switch contact can occur.
Typical device-to-device matching is process lot dependent and
may vary by up to ±30%. Since the resistance element is proc-
essed in thin-film technology, the temperature coefficient of
R
AB
is only 45 ppm/°C.
I
2
C COMPATIBLE 2-WIRE SERIAL BUS
The first byte of the AD5246 is a slave address byte (see Table 6
and Table 7). It has a 7-bit slave address and an R/
W
bit. The
seven MSBs of the slave address are 0101110 followed by 0
for a write command or 1 to place the device in read mode.
The 2-wire I
2
C serial bus protocol operates as follows:
1. The master initiates data transfer by establishing a START
condition, which is when a high-to-low transition on the
SDA line occurs while SCL is high (see Figure 27). The
following byte is the slave address byte, which consists of
the 7-bit slave address followed by an R/
W
bit (this bit
determines whether data will be read from or written to
the slave device).
The slave whose address corresponds to the transmitted
address responds by pulling the SDA line low during the
ninth clock pulse (this is termed the acknowledge bit).
At this stage, all other devices on the bus remain idle while
the selected device waits for data to be written to or read
from its serial register. If the R/
W
bit is high, the master
reads from the slave device. Conversely, if the R/
W
bit is
low, the master writes to the slave device.
2. In write mode, after acknowledgement of the slave address
byte, the next byte is the data byte. Data is transmitted over
the serial bus in sequences of nine clock pulses (eight data
bits followed by an acknowledge bit). The transitions on
the SDA line must occur during the low period of SCL and
remain stable during the high period of SCL (see Table 6).
3. In read mode, after acknowledgment of the slave address
byte, data is received over the serial bus in sequences of
nine clock pulses (a slight difference from the write mode
where eight data bits are followed by an acknowledge bit).
Similarly, the transitions on the SDA line must occur
during the low period of SCL and remain stable during
the high period of SCL (see Figure 28).
4. When all data bits have been read or written, a STOP
condition is established by the master. A STOP condition
is defined as a low-to-high transition on the SDA line while
SCL is high. In write mode, the master pulls the SDA line
high during the tenth clock pulse to establish a STOP
condition (see Figure 27). In read mode, the master issues
a No Acknowledge for the ninth clock pulse (that is, the
SDA line remains high). The master then brings the SDA
line low before the tenth clock pulse, which goes high to
establish a STOP condition (see Figure 28).
