EDN Access--09.12.97 Digitally control gain over a ±40-dB range

-September 12, 1997

September 12, 1997

Digitally control gain over a ±40-dB range

Mark Shill, Burr-Brown Corp, Tucson AZ

Digitally controlled amplifiers typically take the form of programmable-gain amplifiers (PGAs). Though useful for many applications, PGAs have the disadvantage of large, discrete gain steps, such as gains of 1, 2, 4, and 8. For higher resolution gain steps, you must use a different approach. The circuit in Figure 1 can digitally control an amplifier's gain over a ±40-dB range. The circuit has a programmable-gain resolution of 1.25 dB and can attenuate or amplify the input signal, depending on the digital code you apply. You could use this high-resolution PGA as the front end of an ADC and thus obtain increased dynamic range. The circuit is also useful as a digitally controlled audio preamplifier.

The circuit is based on the VCA610 voltage-controlled amplifier, IC3. This IC amplifies or attenuates the differential signal between input pins 1 and 8, depending on the voltage you apply to the VC gain-control, Pin 3. The equation relating the gain of the VCA610 to VC is A=10-2(VC+1). VC can vary linearly over the range of 0 to -2V, representing an amplifier-gain range of -40 to +40 dB, respectively. The circuit uses a 6-bit digital word to set the gain of the VCA610. A simple binary DAC converts the digital gain-control code to the corresponding VC control voltage.

The DAC comprises op-amp IC2, binary-weighted resistors R1 through R6, and feedback resistor R7. IC2, configured in inverting mode, drives IC3's gain-control pin. IC1, a CMOS noninverting driver, buffers the digital input code and provides simple reference-voltage levels of 0 and 5V for the binary-weighted DAC resistors. Each of IC1's outputs typically has an output impedance lower than 50 ohms to either ground or the 5V supply. To minimize the DAC resistors' loading effect on IC1's outputs, the values for R1 through R6 are such that the most-significant-bit (MSB) resistor, R1, is 10 kilo-ohms, and resistors R2 through R6 are binary multiples of 10 kilo-ohms.

To further ensure that IC1 can pull up the MSB resistor, R1, to near 5V, two outputs connect in parallel to provide increased drive. If you use ±1% or better resistors for R1 through R6, the DAC produces a monotonic output at 6 bits. IC2 has a maximum offset voltage of ±0.5 mV and has FET inputs whose bias currents cause no additional offset error for the DAC. R7 is one-fifth of R1, or 2 kilo-ohms, setting the 0 to -2V (actually, 0 to -1.96875V) transfer range of the DAC. The positive full-scale gain of the amplifier is 40 dB minus one least significant bit (LSB), or 38.75 dB. You can adjust the value of R7 to change the range of the DAC, as long as the voltage to the VC pin remains approximately 0 to -2V.

Table 1 shows the relationship between the digital code to the resistor DAC and IC3's control voltage and gain. For a 6-bit DAC, the gain-step increment, or least significant bit (LSB,) is 1.25 dB (LSB=80 dB/26). For increased accuracy and resolution, you can use a 12-bit DAC (Figure 2). Connecting pins 2, 3, and 9 configures DAC IC1 for an output range of 0 to 10V. The DAC813 has internal data latches for the 12-bit digital-input code, which you write to the DAC by momentarily taking the WR signal low. The circuit uses only the first 10 bits of the DAC, providing a gain-step resolution of 0.078 dB (LSB=80 dB/210), or approximately 1%. Op-amp stage IC2 inverts and attenuates the 0 to 10V range of the DAC to a corresponding range of 0 to -2V. (DI #2084)

Table 1 -- gain and v C vs digital input
DAC code VC
000000 0 0.01 -40
010000 -0.5 0.1 -20
100000 -1 1 0
110000 -1.5 10 20
111111 -1.969 86.6 38.75
Figure 1
A simple DAC and a voltage-controlled amplifier allow you to control gain over an 80-dB dynamic range.
Figure 2
Using 10 bits of a 12-bit DAC quadruples the gain resolution of the circuit in Figure 1.

| EDN Access | Feedback | Table of Contents |

Copyright c 1997 EDN Magazine, EDN Access . EDN is a registered trademark of Reed Properties Inc, used under license. EDN is published by Cahners Publishing Company , a unit of Reed Elsevier Inc.



Loading comments...

Write a Comment

To comment please Log In