DS1667
Digital Resistor with OP AMP
www.dalsemi.com
FEATURES
Two digitally controlled 256-position
potentiometers
Serial port provides means for setting and
reading both potentiometers
Resistors can be connected in series to
provide additional resolution
Default wiper position on power up is 50%
Resistive elements are temperature-
compensated to +20% end to end
Two high-gain, wide bandwidth operational
amplifiers
Low power CMOS design
Applications include analog-to-digital and
digital-to-analog converters, variable
oscillators, and variable gain amplifiers
20-pin DIP package or optional 20-pin SOIC
surface mount package
Operating temperature range
- Commercial: 0°C to 70°C
Resistance values:
RESOLUTION -3 dB POINT
DS1667-10: 10k 39 ohms
1.1 MHz
DS1667-50: 50k 195 ohms
200 kHz
DS1667-100: 100k 390 ohms 100 kHz
PIN ASSIGNMENT
NINV0
INV0
V
B
W1
H1
L1
RST
CLK
OUT1
GND
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
V
CC
OUT0
SOUT
W0
H0
L0
COUT
DQ
INVI
NINVI
20-Pin DIP (300-mil) and 20-Pin SOIC
See Mech. Drawings Section
PIN DESCRIPTION
V
CC
GND
L0, L1
H0, H1
W0, W1
V
B
SOUT
RST
DQ
CLK
COUT
NINV0, NINVI
INV0, INVI
OUT0, OUT1
- +5-Volt Supply
- Ground
- Low End of Resistor
- High End of Resistor
- Wiper End of Resistor
- Substrate Bias and OP
AMP Negative Supply
- Wiper for Stacked
Configuration
- Serial Port Reset Input
- Serial Port Input/Output
- Serial Port Clock Input
- Cascade Serial Port
Output
- Noninverting OP AMP
Input
- Inverting OP AMP Input
- OP AMP Outputs
DESCRIPTION
The DS1667 is a dual-solid state potentiometer that is adjustable by digitally selected resistive elements.
Each potentiometer is composed of 256 resistive elements. Between each resistive section of each
potentiometer are tap points accessible to the wiper. The position of the wiper on the resistive array is set
by an 8-bit register that controls which tap point is connected to the wiper output. Each 8-bit register can
be read or written by sending or receiving data bits over a 3-wire serial port. In addition, the resistors can
be stacked such that a single potentiometer of 512 sections results. When two separate potentiometers are
used, the resolution of the DS1667 is equal to the resistance value divided by 256. When the
potentiometers are stacked end to end, the resistance value is doubled while the resolution remains the
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032000
DS1667
same. The DS1667 also contains two high gain wide bandwidth operational amplifiers. Each amplifier
has both the inverting and non-inverting inputs and the output available for user configuration. The
operational amplifiers can be paired with the resistive elements to perform such functions as analog to
digital conversion, digital to analog conversion, variable gain amplifiers, and variable oscillators.
OPERATION - DIGITAL RESISTOR SECTION
The DS1667 contains two potentiometers, each of which has its wiper set by a value contained in an 8-bit
register (see Figure 1). Each potentiometer consists of 256 resistors of equal value with tap points
between each resistor and at the low end.
In addition, the potentiometer can be stacked by connecting them in series such that the high end of
potentiometer 0 is connected to the low end of potentiometer 1. When stacking potentiometers, the stack
select bit is used to select which potentiometer wiper will appear at the stack multiplexer output (SOUT).
A zero written to the stack multiplexer will connect wiper 0 to the SOUT pin. This wiper will determine
which of the 256 bottom taps of the stacked potentiometer is selected. When a 1 is written to the stack
multiplexer, wiper 1 is selected and one of the upper 256 taps of the stacked potentiometer is presented at
the SOUT pin.
BLOCK DIAGRAM
Figure 1
Information is written to and read from the wiper 0 and wiper 1 registers and the stack select bit via the
17-bit I/O shift register. The I/O shift register is serially loaded by a 3-wire serial port consisting of
RST
,
DQ, and CLK. It is updated by transferring all 17 bits (Figure 2). Data can be entered into the 17-bit shift
register only when the
RST
input is at a high level. While at a high level, the
RST
function allows serial
entry of data via the D/Q pin. The potentiometers always maintain their previous value until
RST
is
taken to a low level, which terminates data transfer. While
RST
input is low, the DQ and CLK inputs are
ignored.
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DS1667
Valid data is entered into the I/O shift register while
RST
is high on the low-to-high transition of the
CLK input. Data input on the DQ pin can be changed while the clock input is high or low, but only data
meeting the setup requirements will enter the shift register. Data is always entered starting with the value
of the stack select bit. The next 8 bits to be entered are those specifying the wiper 1 setting. The MSB of
these 8 bits is sent first. The next 8 bits to be entered are those specifying the wiper 0 setting, sent MSB
first. The 17
th
bit to be entered, therefore, will be the least significant bit of the wiper 0 setting. If fewer
than 17 bits are entered, the value of the potentiometer settings will result from the number of bits that
were entered plus the remaining bits of the old value shifted over by the number of bits sent. If more than
17 bits are sent, only the last 17 bits are left in the shift register. Therefore, sending other than 17 bits can
produce indeterminate potentiometer settings.
As bits are entered into the shift register, the previous value is shifted out bit by bit on the cascade serial
port pin (COUT). By connecting the COUT pin to the DQ pin of a second DS1667, multiple devices can
be daisy chained together as shown in Figure 3.
When connecting multiple devices, the total number of bits sent is always 17 times the number of
DS1667s in the daisy chain. In applications where it is desirable to read the settings of potentiometers, the
COUT pin of the last device connected in a daisy chain must be connected back to the DQ input of the
first device through a resistor with a value of 1k to 10k. This resistor provides isolation between COUT
and DQ when writing to the device (see Figure 3).
When reading data, the DQ line is left floating by the reading device. When
RST
is held low, bit 17 is
always present on the COUT pin, which is fed back to the input DQ pin through the resistor (see Figure
4). This data bit can now be read by the reading device. The
RST
pin is then transitioned high to initiate a
data transfer. When the CLK input transitions low to high, bit 17 is loaded into the first position of the
I/O shift register and bit 16 becomes present on COUT and DQ. After 17 bits (or 17 times the number of
devices for a daisy chain), the data has shifted completely around and back to its original position. When
RST
is transitioned back low to end data transfer, the value (the same as before the read occurred) is
loaded into the wiper 0 and wiper 1 registers and the stack select bit.
When power is applied to the DS1667, the device always has the wiper settings at half position and the
stack select bit is at 0.
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DS1667
WRITING DATA
Figure 2
CASCADING MULTIPLE DEVICES
Figure 3
READING DATA
Figure 4
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DS1667
DS1667 LINEARITY MEASUREMENTS
An important specification for the DS1667 is linearity, that is, for a given digital input, how close the
analog output is to that which is expected.
The test circuit used to measure the linearity of the DS1667 is shown in Figure 5. Note that to get an
accurate output voltage it is necessary to assure that the output current is 0, in order to negate the effects
of wiper impedance RW, which is typically 400 ohms. For any given setting N for the pot, the expected
voltage output at SOUT is:
VO = -5 + [10 X (N/256)] (in volts)
Absolute linearity is a comparison of the actual measured output voltage versus the expected value given
by the equation above and is given in terms of an LSB, which is the change in expected output when the
digital input is incremented by 1. In this case the LSB is 10/256 or 0.03906 volts. The equation for the
absolute linearity of the DS1667 is:
V
O
(actual) - V
O
(expected)
LSB
= AL (in LSBs)
The specification for absolute linearity of the DS1667 is + 1 LSB typical.
Relative linearity is a comparison of the difference of actual output voltages of two successive taps and
the difference of the expected output voltages of two successive taps. The expected difference of output
voltages is 1 LSB or 0.03906V for the measurement system of Figure 5. Relative linearity is expressed in
terms of an LSB and is given by the equation:
∆
V
O
(actual) - LSB
LSB
= RL
The specification for relative linearity of the DS1667 is ± 0.5 LSB typical.
Figure 6 is a plot of absolute linearity (AL) and relative linearity (RL) versus wiper setting for a typical
DS1667 at 25°C.
DESCRIPTION AND OPERATION - OP AMP SECTION
The DS1667 contains two operational amplifiers which are ideal for operation from a single 5V supply
and ground or from +5V supplies (see Figure 1). An internal resistor divider defines the internal reference
of the op amp to be halfway between the power supplies, i.e.:
V
DD
+
V
B
2
For optimal performance, choose analog ground to be this value. The operational amplifiers feature rail to
rail output swing in addition to an input common mode range that includes the positive rail. Performance
features include broad band noise immunity as well as voltage gain into realistic loads specified at both
600 ohms and 2k ohms. High voltage gain is produced with low input offset voltage and low offset
voltage drift. Current consumption is less than 1.9 mA per amplifier and the device is virtually immune to
latchup.
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