Design Note 116: Micropower 12-Bit ADCs Shrink Board Space

Introduction

The LTC1286/LTC1298 are serial interfaced, micropower 12-bit analog-to-digital converters. In the realm of 12-bit ADCs they bring a new low in power dissipation and the small size of an SO-8 package to low cost, battery-powered electronic products. These micropower devices consume just 250μA (LTC1286) and 340μA (LTC1298) at full conversion speed and feature autoshutdown.

Many portable and battery-powered systems require internal analog-to-digital conversion. Some, such as pen-based computers, have ADCs at their very cores digitizing the pen screen. Other systems use ADCs more peripherally to monitor voltages or other parameters inside the equipment. Regardless of the use, it has been difficult to obtain small ADCs at power levels and prices that are low enough. The LTC1286/LTC1298 meet these low power dissipation and package size needs.

Micropower and 12-Bits in an SO-8 Package

The LTC1286/LTC1298 are the latest members of the growing family of SO-8 packaged parts (Table 1). As the first of their kind in SO-8 packages, these are improvements to the 8-bit micropower LTC1096/LTC1098 ADCs. The LTC1286/LTC1298 use a successive approximation register (SAR) architecture. Both converters contain sample-and-holds and serial data I/O. The LTC1286 has a fully differential analog input and the LTC1298 has a two input multiplexer. While running at a full speed conversion rate of 12.5ksps, the LTC1286 consumes only 250μA from a single 5V supply voltage. The device automatically shuts down to 1nA (typ) when not converting. Figure 1 shows how this automatically reduces power at lower sample rates. At a 1ksps conversion rate, the supply current drops to just 20μA (typ). Battery-powered designs will benefit tremendously from this user transparent automatic power dissipation optimization.

The DC specifications include an excellent differential linearity error of 0.75LSB (max) and no missing codes. Both are guaranteed over the operating temperature range. Pen screen and other monitoring applications benefit greatly from these tight specifications.

The attractiveness of the LTC1286/LTC1298’s small SO-8 design is further enhanced by the use of just one surface mount bypass capacitor (1μF or less). Figure 2 shows a typical connection to a microcontroller’s serial port. For ratiometric applications that require no external reference voltage, the LTC1286/LTC1298’s reference input is tied to signal source’s drive voltage. With their very low supply current requirements, the ADCs can even be powered directly from an external voltage reference. This eliminates the need for a separate voltage regulator.

The LTC1286/LTC1298 contain everything required except an internal reference (not needed by many applications) keeping systems costs low. The serial interface makes a very space efficient interface and significantly reduces cost of applications requiring isolation. The ADC’s high input impedance eliminates the need for buffer amplifiers. All of these features, combined with a very attractive price, make the LTC1286/LTC1298 ideal for new designs.

Resistive Touchscreen Interface

Figure 3 shows the LTC1298 in a 4-wire resistive touchscreen application. Transistor pairs Q1 and Q3, Q2 and Q4 apply 5V and ground to the X axis and Y axis, respectively. The LTC1298 (U1), with its 2-channel multiplexer, digitizes the voltage generated by each axis and transmits the conversion results to the system’s processor through a serial interface. RC combinations R1C1, R2C2 and R3C3 form lowpass filters that attenuate noise from possible sources such as the processor clock, switching power supplies and bus signals. Inverter U2A is used to detect screen contact both during a conversion sequence and to trigger its start. Using the single channel LTC1286, 5-wire resistive touchscreens are as easily accommodated.

著者

Kevin Hoskins