DS1856E-050 Maxim Integrated Products, DS1856E-050 Datasheet - Page 25

• Manufacturer Part Number: DS1856E-050
• Description: Digital Potentiometer ICs
• Manufacturer: Maxim Integrated Products

Specifications of DS1856E-050

• Number Of Pots: Dual
• Taps Per Pot: 256
• Resistance: 50 KOhms
• Wiper Memory: Non Volatile
• Digital Interface: Serial (2-Wire)
• Operating Supply Voltage: 3.3 V, 5 V
• Supply Current: 2 mA
• Maximum Operating Temperature: + 95 C
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• Supply Voltage (max): 5.5 V
• Supply Voltage (min): 2.85 V
• Package / Case: TSSOP-16
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The following equation can be used to determine which
resistor position setting, 00h to FFh, should be written in
the LUT to achieve a given resistance at a specific tem-
perature.
R = the resistance desired at the output terminal
C = temperature in degrees Celsius
u, v, w, x 1 , x 0 , y, z, and
the corresponding look-up tables. The variable x from the
equation above is separated into x 1 (the MSB of x) and x 0
(the LSB of x). Their addresses and LSB values are given
below. The variable y is assigned a value. All other vari-
ables are unsigned. Resistor 0 variables are found in
Table 04, and Resistor 1 variables are found in Table 05.
When shipped from the factory, all other memory loca-
tions in the LUTs are programmed to FFh.
Table 8. Calibration Constants
The DS1856 has two methods for scaling an analog
input to a digital result. The two methods are gain and
offset. Each of the inputs (V
MON3) has a unique register for the gain and the offset
found in Table 03h, 92h to 99h, and A2h to A9h.
pos
nally Calibrated Monitors and Password Protection
ADDRESS
Dual, Temperature-Controlled Resistors with Inter-
FAh
FBh
FCh
FDh
F8h
F9h
FEh
FFh
Programming the Look-up Table (LUT)
R C
,
R u x
x x
VARIABLE
x
x
w
u
v
y
z
1
1
0
1
y x C
v x C
are calculated values found in
2E-6 (signed)
8E-6 (signed) for -025 version
4E-6 (signed) for -030 version
Internal Calibration
CC
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, MON1, MON2, and
z x C
w x C
____________________________________________________________________
100E-9
20E-6
10E-9
LSB
2
2
2
2
-7
-2
0
1
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2
2
To scale the gain and offset of the converter for a spe-
cific input, you must first know the relationship between
the analog input and the expected digital result. The
input that would produce a digital result of all zeros is
the null value (normally this input is GND). The input
that would produce a digital result of all ones is the full-
scale (FS) value. The FS value is also found by multiply-
ing an all-ones digital answer by the weighted LSB
(e.g., since the digital reading is a 16-bit register, let us
assume that the LSB of the lowest weighted bit is
50µV, then the FS value is 65,535 x 50µV = 3.27675V).
A binary search is used to scale the gain of the con-
verter. This requires forcing two known voltages to the
input pin. It is preferred that one of the forced voltages
is the null input and the other is 90% of FS. Since the
LSB of the least significant bit in the digital reading reg-
ister is known, the expected digital results are also
known for both inputs (null/LSB = CNT1 and 90%FS/
LSB = CNT2).
The user might not directly force a voltage on the input.
Instead they have a circuit that transforms light, fre-
quency, power, or current to a voltage that is the input
to the DS1856. In this situation, the user does not need
to know the relationship of voltage to expected digital
result but instead knows the relationship of light, fre-
quency, power, or current to the expected digital result.
An explanation of the binary search used to scale the
gain is best served with the following example pseudo-
code:
/* Assume that the null input is 0.5V. */
/* In addition, the requirement for LSB is 50µV. */
/* Thus the null input 0.5V and the 90% of FS input is
2.949075V. */
See the Right-Shifting section);
FS = 65535 x 50E-6;
CNT1 = 0.5 / 50E-6;
CNT2 = 0.90 x FS / 50E-6;
Set the trim-offset-register to zero;
Set Right-Shift register to zero (typically zero.
gain_result = 0h;
Clamp = FFF8h/2^(Right_Shift_Register);
For n = 15 down to 0
begin
/* 3.27675 */
/* 10000 */
/* 58981.5 */
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