DS1667 Digital Potentiometer with Operational Amplifier

The DS1667 contains two integrated circuit potentiometers, which can be adjusted by selecting resistor elements by counting. Each potentiometer consists of 256 resistor elements, and each resistor segment has a tap that can be connected to the cursor. The position of the cursor in the resistor array is set by an 8-bit register, which controls which tap the output of the cursor is connected to. Each 8-bit register sends or receives data bits through a 3-wire serial port for read/write operations. In addition, two potentiometers can be connected in series to form a single potentiometer with 512 resistor segments. When the two potentiometers are used separately, the resolution of the DS1667 is equal to the resistance value of the resistor divided by 256. When the potentiometers are connected in series, the resistance value of the resistor is twice the original, but the resolution remains unchanged. The DS1667 contains two high-gain broadband operational amplifiers, each of which has a non-inverting and inverting input terminal and an output terminal for user design and use. The operational amplifier and the potentiometer work together to complete certain functions, such as analog/digital conversion, digital/analog conversion, variable gain amplifier, variable frequency oscillator, etc.

2 Pin Arrangement and Description

　　VCC + 5V power supply

　　GND

　　L0, L1 potentiometer low end

　　DQ serial port input/output

　　H0, H1 potentiometer high end

　　CLK serial port clock input

　　W0, W1 potentiometer wiper terminal

　　COUT cascade serial port output

　　VB is the negative power supply terminal of the operational amplifier

　　NINV0, NINV1 operational amplifier non-inverting input terminal

　　SOUT The cursor output terminal of the stack structure

　　INV0, INV1 operational amplifier inverting input

　　RST serial port reset input terminal

　　OUT0, OUT1 operational amplifier output

3 Main Features

　　The main features of this digital potentiometer are as follows:

　　Two digitally controlled 256-position potentiometers

　　The serial port provides a way to set and read the two potentiometers

　　Two potentiometers in series can provide additional resolution

　　The default position of the wiper at power-on is 1/2 of the resistor value.

　　The temperature compensation of the resistor element between the two ends of the potentiometer can reach ±20%

　　Two high-gain, wideband operational amplifiers

　　Low power CMOS design

　　Applications: analog/digital conversion and digital/analog conversion, variable frequency oscillators, variable gain amplifiers, etc.

　　20-pin dual in-line package (DIP), 20-pin SOIC surface mount

　　Operating temperature range: 0℃～70℃

　　·Resistors Resistance Resolution of Resistors - 3dB Point

DS1667－1010Ω39Ω1.1MHz

DS1667－5050Ω195Ω200.0kHz

DS1667－100100Ω390Ω100.0kHz

4 Working principle of digital potentiometer part

　　The principle block diagram of the DS1667 digital potentiometer is shown in Figure 2. As shown in Figure 2, the DS1667 contains two potentiometers, each with its own cursor, which is set by a value contained in an 8-bit register. Each potentiometer consists of 256 resistors of equal resistance, which are connected to each other and to the end resistor with taps.

　　Alternatively, the potentiometers can be stacked in series, that is, the high end of potentiometer 0 is connected to the low end of potentiometer 1, as a stacked potentiometer, and the stack select bit is used to select which potentiometer's wiper will appear at the multiplexer output SOUT. If a 0 is written to the stack multiplexer, wiper 0 will be connected to the SOUT pin. This wiper will determine which bit is selected from the bottom 256 taps of the stacked potentiometer. If a 1 is written to the stack multiplexer, wiper 1 will be selected and one of the 256 taps on the top of the stacked potentiometer will be connected to the SOUT pin.

　　Data can be read or written from the cursor 0 and cursor 1 registers and the stack select bit through the 17-bit I/O shift register. The I/O shift register is a load for the 3-wire serial port, which consists of RST, DQ, and CLK. It modifies data by transmitting 17 bits. Data is allowed to be written serially through the DQ pin only when the RST input is high. The potentiometer always maintains the previous value before the RST terminal becomes low. The value of the potentiometer will change only after RST becomes low. When the RST input is low, the DQ and CLK inputs have no effect.

　　When RST is high, the CLK input transitions from low to high and valid data is written to the I/O shift register. The input data at the DQ pin can change regardless of whether the clock input is high or low, and the value of the DQ pin is only sent to the shift register when the setting requirements are met. Data writing always starts with the value of the stack select bit. The next 8 bits sent in are the value of the wiper setting for potentiometer 1. The most significant bit of these 8 bits of data is sent first. The next 8 bits are the value of the wiper setting for potentiometer 0. The most significant bit is sent first. The 17th bit of data sent in is the least significant bit of the wiper 0 setting. If the data written in is less than 17 bits, the value of the potentiometer setting will be the data written in plus the previously unconverted reserved bits. If the data written in is more than 17 bits, the last 17 bits of data are left in the shift register. Therefore, if the data sent in is not 17 bits, it will cause the potentiometer setting to be inaccurate.

　　When multiple bits of data are written into the shift register, the previous data is shifted out bit by bit through the cascade serial port pin COUT. By connecting the COUT of one DS1667 to the DQ pin of another DS1667, multiple potentiometers can be connected in series like a chain, as shown in Figure 3.

When reading data, the DQ pin is in a floating state. When RST is kept low, bit 17 always appears at the COUT pin, which is fed back to the DQ pin through a resistor (as shown in Figure 4), and the data is read out by the reading device. The RST pin becomes high to start the data transmission. When the CLK input changes from low to high, bit 17 is sent to the first bit of the I/O shift register, and bit 16 appears at the COUT pin and DQ pin. When all 17 bits are transferred, the data has been completely moved to the initial position. When RST changes back to a low level to end the data transfer, the data (similar to the value before the read occurred) is sent to the register of cursor 0, cursor 1, and the stack select bit.

　　For the DS1667, each time power is applied, the potentiometer wiper is set to halfway and the stack select bit is set to zero.

5 Operational Amplifier

　　The DS1667 contains two ideal operational amplifiers, whose operating voltage is 5V or ±5V (as shown in Figure 1). The internal resistor divider sets the internal reference value of the operational amplifier to the average value of the two power supplies, that is, (VDD + VB) / 2. In order to obtain the best working characteristics, this value is selected as the analog ground reference value.

Parameters of 6DS1667

　　(1) Extreme working conditions

　　Voltage of any pin relative to ground: -0.5V～7.0V

When VB=5.5V, the resistor pin voltage is: -5.5V～7.0V

　　VB voltage: -5.5V to ground

　　Working temperature: 0℃～70℃

　　Storage temperature: -55℃～125℃

　　Soldering temperature: 260°C (10 seconds)

　　(2) Recommended DC operating conditions for potentiometers

　　Table 1 shows the working conditions of the potentiometer, the temperature range is 0℃～70℃.

Table 1 DC working conditions of potentiometer

parameter	symbol	Minimum	Typical Value	Maximum	unit
Positive voltage	VCC	+4.5	5.0	5.5	V
Input logic 1	VHI	2.0		VCC＋0.5	V
Input logic 0	VIL	-0.5		+0.8	V
Negative voltage	VB	-5.5		GND	V
Resistor input	L, H, W	VB－0.5		VCC＋0.5	V

　　(3) Main electrical characteristics of operational amplifiers

　　The main electrical characteristics of the operational amplifier are shown in Table 2.

Table 2 Electrical characteristics of operational amplifiers

parameter	symbol	Minimum	Typical Value	Maximum	unit
Input offset voltage	VOS		5	10	V
Input offset voltage drift	VOSD		10		μv/℃
Common Mode Rejection Ratio	CMR		62		dB
Input common mode voltage range	CCCM	VB+1.5		VCC	V
Output Swing	VSWGH		4.7		V
Unity-Gain-Bandwidth Product	GBP		2.5		MHz

7 Application Examples

　　Because DS1667 is digitally controlled and programmable, it is mainly used in automatic control as a fixed gain attenuator, variable gain amplifier and differential amplifier, as shown in Figure 6. It can work more accurately, flexibly and at a higher speed as needed. DS1667 has two independent potentiometers that can be used, and two independent operational amplifiers that can be connected to the potentiometers or used separately.