Digital potentiometer DS1667 with operational amplifier

DS1667 contains two integrated circuit potentiometers, which can be adjusted by the method of selecting resistance elements by counting method. Each potentiometer is composed of 256 resistance elements, and there is a tap that can be connected to the cursor in each resistance section. The position of the cursor in the resistor array is set by an 8-bit register, which controls which tap the output terminal 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 resistance segments. When the two potentiometers are used separately, the resolution of the DS1667 is equal to the resistance of the resistor divided by 256. When the potentiometer is connected in series, the resistance of the resistor is twice that of the original, but the resolution is unchanged. DS1667 contains two high-gain wideband operational amplifiers, each of which has an in-phase and inverting input and an output for user design. Operational amplifiers and potentiometers perform certain functions together, such as analog-to-digital conversion, digital-to-analog conversion, variable gain amplifiers, variable frequency oscillators, and so on.

2 pin arrangement and description

VCC + 5V power supply

GND ground

Low end of L0, L1 potentiometer

DQ serial input / output

H0, H1 potentiometer high-end

CLK serial clock input

W0, W1 potentiometer cursor end

Serial output terminal of COUT cascade

The negative power supply terminal of VB operational amplifier

NINV0, NINV1 operational amplifier non-inverting input

Cursor output of SOUT stack structure

INV0, INV1 operational amplifier inverting input

RST serial reset input

OUT0, OUT1 operational amplifier output

3 main features

The main features of this digital potentiometer are as follows:

· Two digitally controlled 256-bit potentiometers

· Serial port provides setting and reading methods for two potentiometers

· Two potentiometers connected in series can provide additional resolution

Figure 1 Pin arrangement of DS1667

Figure 2 Functional block diagram

· The default position of the cursor at power-on is at the 1/2 position of the resistance of the resistor

· The temperature compensation of the resistance element between the two ends of the potentiometer can reach ± ​​20%

· Two high-gain wideband operational amplifiers

· Low power CMOS design

· Apply A / D conversion and D / A conversion, variable frequency oscillator, variable gain amplifier, etc.

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

· Working temperature range: 0 ℃ ~ 70 ℃

· Resistance of the resistor Resistance resolution of the resistor-3dB point

DS1667-1010Ω39Ω1.1MHz

DS1667-5050Ω195Ω200.0kHz

DS1667-100100Ω390Ω100.0kHz

4 Working principle of the digital potentiometer part

The principle block diagram of the DS1667 digital potentiometer is shown in Figure 2. From Figure 2, it can be seen that the DS1667 contains two potentiometers. Each potentiometer has its own cursor, which is set by a value contained in an 8-bit register. Each potentiometer is composed of 256 resistors with equal resistance, which are connected by taps to each other and the last resistor.

In addition, 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 stack potentiometer, the stack selection bit is used to select which potentiometer cursor will appear in multiple Road output SOUT. If 0 is written to the stack multiplexer, connect cursor 0 to the SOUT pin. This cursor will determine which bit is selected from the 256 taps at the bottom of the stacked potentiometer. If 1 is written to the stack multiplexer, cursor 1 will be selected, and one of the 256 taps on the top of the stack potentiometer is connected to the SOUT pin.

Through the 17-bit I / O shift register, data can be read or written from the cursor 0 and cursor 1 registers and the stack selection bits. The I / O shift register is a 3-wire serial port load, and the 3-wire serial port consists of RST, DQ, and CLK. It modifies the data by transmitting 17 digits. Only when RST input high level, serial write data is allowed through DQ pin. Before the RST terminal becomes low level, the potentiometer always keeps the previous value unchanged. When RST goes low, the value of the potentiometer will change. 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 into the I / O shift register. Regardless of whether the clock input is high or low, the input data of the DQ pin can be changed, and the value of the DQ pin is sent to the shift register only when the setting requirements are met. Data writing always starts with the value of the stack selection bit. The next 8-bit input is the cursor setting value specifying the potentiometer 1, the most significant bit of the 8-bit data is sent first, the next 8 bits are the setting value specifying the cursor of the potentiometer 0, the first is also sent Most significant bit. The 17th data sent is the least significant bit set by cursor 0. If the written data is less than 17 digits, the value set by the potentiometer will be the written data plus the reserved bits that were not previously converted. If the data written is greater than 17 bits, the last 17 bits of data are left in the shift register. Therefore, if the data sent is not 17 bits, the potentiometer setting will be inaccurate.

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

Figure 3 write data

Figure 4 cascading multiple devices

Figure 5 Reading data

Figure 6 programmable differential amplifier

When reading data, the DQ pin is in a floating state. When RST remains low, bit 17 always appears on the COUT pin, which is fed back to the DQ pin through a resistor (see Figure 4), and the data is read out by the reading device. The RST pin goes high to start the data transfer. When the CLK input transitions from low to high, bit 17 is fed into the first bit of the I / O shift register, and bit 16 appears on the COUT pin and DQ pin. When all 17 bits have been transmitted, the data has been completely moved to the initial position. When RST goes back low to end the data transfer, the data (similar to the value before the read occurred) is sent to the cursor 0, cursor 1 registers and stack selection bits.

For the DS1667, every time power is applied, the cursor of the potentiometer is set at half the position, and the stack selection bit is set at zero.

5 Operational amplifier

DS1667 contains two ideal operational amplifiers, its working voltage is 5V or ± 5V (as shown in Figure 1), the internal resistance divider sets the internal reference value of the operational amplifier is the average of the two power supplies, namely (VDD + VB) 2. In order to get the best operating characteristics, select this value as the reference value of the simulation.

6DS1667 parameters

(1) Extreme working conditions

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

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

VB voltage: -5.5V ~ ground

Working temperature: 0 ℃ ~ 70 ℃

Storage temperature: -55 ℃ ~ 125 ℃

Welding temperature: 260 ° C (10 seconds)

(2) Recommended DC operating conditions of the potentiometer

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

Table 1 DC working conditions of potentiometer

Parameter Symbol Min Typical Value Max 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) The main electrical characteristics of the operational amplifier

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

Table 2 Electrical characteristics of operational amplifiers

Parameter Symbol Min Typical Value Max 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, they are mainly used in automatic control, as fixed gain attenuators, variable gain amplifiers and differential amplifiers, as shown in Figure 6. According to the need, it can work more accurately, more flexibly and at higher speed. DS1667 has two independent potentiometers that can be used, and two independent operational amplifiers can be connected to the potentiometer or used separately.

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