EDN Access — 09.12.96 Programmable ramp generator has 14-bit linearit

-September 12, 1996

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Design Ideas:September 12, 1996

Programmable ramp generator has 14-bit linearity

Dan Segarra,
Sipex Corp, Billerica, MA

This linear ramp generator uses a DAC, a transistor array, MOSFET switches, and current-steering techniques to achieve 14-bit linearity (Figure 1). The output current of IC1, a DAC-10, is a product of the digital code and the reference-current input, where IO=(code/1024)x2IR. IR is the output of reference IC6 divided by R1. Q1A, Q1B, and Q1C of the CA3096 transistor array form the current sink and share the IO current from the DAC. Q1A simply scales the current by shunting one-third of IO. The pnp transistors of the same array, Q1D and Q1E, mirror the sink current to form a current source of the same magnitude.

The slope input signal drives optocoupler IC4, which isolates the sensitive analog signals from digital currents. A local regulator, IC3, powers the 74HC04 inverters, IC5A and IC5B, that generate the complementary source and sink signals.

A quad analog-switch array, IC2, provides current steering. When the slope input is high, the sink signal is high. In this case, MOSFET switches IC2A and IC2D are on, and IC2B and IC2C are off. IC2A shunts source current from Q1E to ground, and IC2D steers sink current, drawn by Q1C from the op-amp input, into integrator capacitor C1, so that VOUT increases linearly.

When the slope input is low, the source signal is high. In this case, IC2B and IC2C are on, and IC2A and IC2D are off. Q1E drives source current through IC2B into C1 causing the op-amp output voltage to fall linearly, while Q1C draws sink current from ground through IC2C. With switch S1 in the 2-to-1 position, the source current has twice the amplitude of the sink current, so that the output voltage has twice the slope falling as it does rising.

Collector resistors R2 through R5 reduce any tendency of the bipolar transistors to oscillate. Emitter-degeneration resistors R6 through R10 improve the current matching between transistors. Use of a high-stability capacitor with low dielectric adsorption for C1, such as polystyrene or Teflon, helps to maintain integrator linearity. You may want to include a reset function, such as a FET connected across the feedback capacitor, to set the ramp generator to a good starting place for the next ramp. (DI #1921)

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