概要

The AD8400/AD8402/AD8403 provide a single-, dual-, or quad-channel, 256-position, digitally controlled variable resistor (VR) device.1 These devices perform the same electronic adjustment function as a mechanical potentiometer or variable resistor. The AD8400 contains a single variable resistor in the compact SOIC-8 package. The AD8402 contains two independent variable resistors in space-saving SOIC-14 surface-mount packages. The AD8403 contains four independent variable resistors in 24-lead PDIP, SOIC, and TSSOP packages. Each part contains a fixed resistor with a wiper contact that taps the fixed resistor value at a point determined by the digital code loaded into the controlling serial input register. The resistance between the wiper and either endpoint of the fixed resistor varies linearly with respect to the digital code transferred into the VR latch. Each variable resistor offers a completely programmable value of resistance between the A terminal and the wiper or the B terminal and the wiper. The fixed A-to-B terminal resistance of 1 kΩ, 10 kΩ, 50 kΩ, or 100 kΩ has a ±1% channel-to-channel matching tolerance with a nominal temperature coefficient of 500 ppm/°C.
A unique switching circuit minimizes the high glitch inherent in traditional switched resistor designs, avoiding any make-before-break or break-before-make operation.
Each VR has its own VR latch that holds its programmed resistance value. These VR latches are updated from an SPI compatible, serial-to-parallel shift register that is loaded from a standard 3-wire, serial-input digital interface. Ten data bits make up the data-word clocked into the serial input register.
The data-word is decoded where the first two bits determine the address of the VR latch to be loaded, and the last eight bits are the data. A serial data output pin at the opposite end of the serial register allows simple daisy chaining in multiple VR applications without additional external decoding logic.
The reset (RS) pin forces the wiper to midscale by loading 80H into the VR latch. The SHDN pin forces the resistor to an end-to-end open-circuit condition on the A terminal and shorts the wiper to the B terminal, achieving a microwatt power shutdown state. When SHDN is returned to logic high, the previous latch settings put the wiper in the same resistance setting prior to shutdown. The digital interface is still active in shutdown so that code changes can be made that will produce new wiper positions when the device is taken out of shutdown.

 

特徴

256-position variable resistance device
Replaces 1, 2, or 4 potentiometers
1 kΩ, 10 kΩ, 50 kΩ, 100 kΩ
Power shutdown—less than 5 μA
3-wire,SPI-compatible serial data input
10 MHz update data loading rate
2.7 V to 5.5 V single-supply operation
自動車用途に適合

 

アプリケーション

Mechanical potentiometer replacement
Programmable filters, delays, time constants
Volume control, panning
Line impedance matching
Power supply adjustment

 

動作理論

The AD8400/AD8402/AD8403 provide a single, dual, and quad channel, 256-position, digitally controlled variable resistor (VR) device. Changing the programmed VR setting is accomplished by clocking in a 10-bit serial data-word into the SDI (Serial Data Input) pin. The format of this data-word is two address bits, MSB first, followed by eight data bits, also MSB first.
The AD8400/AD8402/AD8403 do not have power-on midscale preset, so the wiper can be at any random position at power-up.
However, the AD8402/AD8403 can be reset to midscale by asserting the RS pin, simplifying initial conditions at power-up. Both parts have a power shutdown SHDN pin that places the VR in a zero-power-consumption state where Terminal Ax is open-circuited and the Wiper Wx is connected to Terminal Bx, resulting in the consumption of only the leakage current in the VR. In shutdown mode, the VR latch settings are maintained so that upon returning to the operational mode, the VR settings return to the previous resistance values. The digital interface is still active in shutdown, except that SDO is deactivated. Code changes in the registers can be made during shutdown that will produce new wiper positions when the device is taken out of shutdown.