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AD7376AN100 Datasheet(PDF) 8 Page - Analog Devices

Part No. AD7376AN100
Description  -15 V Operation Digital Potentiometer
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7376AN100 Datasheet(HTML) 8 Page - Analog Devices

 
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AD7376
–8–
REV. 0
0.1V
CODE = OOH
ISW
VSS TO VDD
W
B
DUT
0.1V
ISW
RSW =
Figure 38. Incremental ON Resistance Test Circuit
ICM
VCM
W
B
DUT
VDD
VSS
NC
A
NC
GND
Figure 39. Common-Mode Leakage Current Test Circuit
OPERATION
The AD7376 provides a 128-position digitally-controlled vari-
able resistor (VR) device. Changing the programmed VR set-
tings is accomplished by clocking in a 7-bit serial data word into
the SDI (Serial Data Input) pin, while
CS is active low. When
CS returns high the last seven bits are transferred into the RDAC
latch setting the new wiper position. The exact timing require-
ments are shown in Figure 1.
The AD7376 resets to a midscale by asserting the
RS pin, sim-
plifying initial conditions at power-up. Both parts have a power
shutdown
SHDN pin which places the RDAC in a zero power
consumption state where terminal A is open circuited and the
wiper W is connected to B, resulting in only leakage currents
being consumed in the VR structure. In shutdown mode the
VR latch settings are maintained so that, returning to opera-
tional mode from power shutdown, the VR settings return to
their previous resistance values.
D6
D5
D4
D3
D2
D1
D0
RDAC
LATCH
&
DECODER
RS
RS
RS
RS
SHDN
A
W
B
RS = RNOMINAL/128
Figure 40. AD7376 Equivalent RDAC Circuit
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between terminals A and
B are available with values of 10 k
Ω, 50 kΩ, 100 kΩ and 1 MΩ.
The final three characters of the part number determine the
nominal resistance value, e.g., 10 k
Ω = 10; 50 kΩ = 50; 100 kΩ
= 100; 1 M
Ω = 1M. The nominal resistance (R
AB) of the VR
has 128 contact points accessed by the wiper terminal, plus the
B terminal contact. The 7-bit data word in the RDAC latch is
decoded to select one of the 128 possible settings. The wiper’s first
connection starts at the B terminal for data 00H. This B–termi-
nal connection has a wiper contact resistance of 120
Ω. The
second connection (10 k
Ω part) is the first tap point located
at 198
Ω (= R
BA [nominal resistance]/128 + RW = 78 Ω + 120 Ω)
for data 01H. The third connection is the next tap point repre-
senting 156 + 120 = 276
Ω for data 02
H. Each LSB data value
increase moves the wiper up the resistor ladder until the last tap
point is reached at 10041
Ω. The wiper does not directly con-
nect to the B terminal. See Figure 40 for a simplified diagram of
the equivalent RDAC circuit.
The general transfer equation that determines the digitally pro-
grammed output resistance between W and B is:
RWB(D) = (D)/128
× R
BA + RW
(1)
where D is the data contained in the 7-bit VR latch, and RBA is
the nominal end-to-end resistance.
For example, when VB = 0 V and A–terminal is open circuit, the
following output resistance values will be set for the following
VR latch codes (applies to the 10 k
Ω potentiometer).
Table I.
DRWB
(DEC)
( )
Output State
127
10041
Full-Scale
64
5120
Midscale (
RS = 0 Condition)
1
276
1 LSB
0
198
Zero-Scale (Wiper Contact Resistance)
Note that in the zero-scale condition a finite wiper resistance of
120
Ω is present. Care should be taken to limit the current flow
between W and B in this state to a maximum value of 5 mA to
avoid degradation or possible destruction of the internal switch
contact.
Like the mechanical potentiometer the RDAC replaces, it is
totally symmetrical. The resistance between the wiper W and
terminal A also produces a digitally controlled resistance RWA.
When these terminals are used the B–terminal should be tied to
the wiper. Setting the resistance value for RWA starts at a maxi-
mum value of resistance and decreases as the data loaded in the
latch is increased in value. The general transfer equation for this
operation is:
RWA(D) = (128-D)/128
× R
BA + RW
(2)
where D is the data contained in the 7-bit RDAC latch, and RBA
is the nominal end-to-end resistance. For example, when VA = 0 V
and B–terminal is tied to the wiper W the following output
resistance values will be set for the following RDAC latch codes.


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