Analogue, Mixed Signal, LSI


Versatile programmable amplifiers use digital potentiometers with nonvolatile memory

28 August 2002 Analogue, Mixed Signal, LSI

In concept, an op-amp and a mechanical potentiometer can easily be combined to form an adjustable-gain amplifier, useful where electronic adjustments are needed. However, this combination is often unfeasible because of the potentiometer's limited resolution, poor temperature coefficient, high resistance drift over time, and the difficulties of remote adjustment. A family of Analog Devices' digital potentiometers with nonvolatile memory can replace their mechanical counterparts and make these circuits practical.

The features offered by the AD523x digital potentiometers are extensive, but controlling them is relatively simple. A menu of 16 instructions provides functions such as setting the 'wiper' position, storing the wiper setting, reading back the wiper position, increment/decrement-by-one, and increment/decrement-by-6 dB (log taper adjustment). A write-protect pin provides memory protection, and additional nonvolatile memory is available for storing user information. Members of the AD523x family are available with one to four channels, 64 to 1024-position resolutions and 10-250 kOhm resistance.

For operation, the user must furnish a power supply and digital controls (via 3-wire SPI serial communications) to realise the flexibility and reliability of these digital potentiometers. The parts will operate with a single supply, but dual supplies are required for AC or bipolar operation. A microcontroller or DSP is usually available to generate the control signals, but the nonvolatile memory enables users to permanently program the devices in the factory without ever having to readjust the settings in the field. This allows the devices to be controlled with PCs using programs such as Visual Basic, C, or LabView. It is unnecessary, therefore, to include a controller in these applications unless dynamic adjustment is needed. Evaluation boards are available.

Basic programmable amplifier

Figure 1. Programmable amplifier/attenuator
Figure 1. Programmable amplifier/attenuator

In the circuit of Figure 1, the gain (negative) is simply the ratio of the two terminal resistances, and the output voltage is:

Where:

RAB = nominal end-to-end terminal resistance

RWB = terminal resistance, W to B,

RWB = RAB _D/2N

RWA = terminal resistance, W to A,

RWA = RABM - RWB = RAB

X (1-D/2N)

D = Base-10 equivalent of the binary word

N = number of bits

The gain expression implies a balanced quasi-logarithmic characteristic. This inverting configuration is useful because it makes available a wide range of gains, from very small to very large, with unity near half-scale (MSB). Because the resistors are fabricated on a single monolithic chip, resistance ratios are inherently matched, and the circuit can yield a temperature coefficient as low as 50 ppm/°C if using an AD5235. The grounded +input minimises common-mode input errors. This circuit is a basic building block that suits many applications, especially where small signals are present and where high gain is required.

Bipolar programmable-gain amplifier with linear step adjustment

In the basic circuit of Figure 1, the output is always inverted with respect to the input, regardless of whether the circuit is providing gain or attenuation. The change in gain as the potentiometer is incremented is nonlinear. While this is useful in some cases, other applications may call for bipolar gain and/or a simple linear relationship. For example, motors need to rotate freely in both forward and reverse directions, thermal electric coolers heat or cool lasers, depending upon the direction of the current flow, and LCD panels require bipolar voltages for the contrast and brightness controls. In the most general case, to create a bipolar drive with arbitrary end points and linear step adjustment, a dual digital potentiometer, such as the AD5232, can be applied as shown in Figure 2. The output, Vo, can now be linearly programmed to amplify voltages between +VI and -K x VI, where K is the ratio of the two terminal resistances of U1 (Equation 1). A2 provides buffered amplification for VW2, minimising the influence of the wiper resistance.

Figure 2. Bipolar programmable-gain amplifier with linear step adjustment
Figure 2. Bipolar programmable-gain amplifier with linear step adjustment

The transfer function in Figure 2 is:

In the simpler (and much more usual) case, where K = 1, a single digital potentiometer, such as the AD5231, is used in location U2, and U1 is replaced by a matched pair of resistors to apply VI and -VI at the end terminals of the digital pot. The relationship will be

Table 1 shows the result of adjusting D, with A2 configured: as a unity-gain follower (R1 = [infinity], R2 = 0), with a gain of 2 (R1 = R2), and with a gain of 10 (R2 = 9 x R1). The result is a bipolar amplifier with linearly programmable gain and 256-step resolution.

Table 1. Circuit gain versus D
Table 1. Circuit gain versus D

As implied in Figure 2, R1/R2 and/or R3/R4 can optionally be replaced by digital potentiometers - if tight temperature-coefficient matching and very high gains are desired. If discrete resistors are used, resistor matching is imperative.

The wiper resistance of the digital potentiometer is the on-resistance of the internal solid-state switches, typically 50-100 Ohm. This is relatively small when compared with the nominal resistance RAB, but the wiper resistance approximately doubles over the operating temperature range and can become the major source of error when the device is programmed to operate at low values. The wiper terminal of the potentiometer should always be connected to a high-impedance node, such as the input terminal of an op-amp, as shown in the above circuits. The OP1177 family, the fourth generation of the industry-standard OP07, was chosen for its low offset and low bias-current characteristics. This minimises the effects of wiper resistance on the voltage divider ratio at the tap point.

The circuit in Figure 2 can also serve as a programmable gain amplifier (PGA) for A/D converters. A/D converters often need to handle dynamic input ranges from a few millivolts to several volts. The AD7723 16-bit, 1,2 MSa/s sigma-delta ADC, for example, has an input voltage limit of ±2 V when using the internal reference. A PGA can be applied as a front-end signal-conditioning device that scales millivolt-level signals to the input range of the converter. If the input voltage range is higher than ±2,7 V, discrete resistors are needed in series with U1 due to the 5,5 V maximum voltage limitation of the AD523x potentiometers. Figure 3 shows the use of series resistors in a high-voltage PGA application of the configuration of Figure 2. As noted earlier, resistor matching is essential.

Figure 3. PGA handles high input voltage
Figure 3. PGA handles high input voltage

By configuring the AD523x nonvolatile memory digital potentiometers and op-amps shown in this article, the user can easily design a versatile amplifier with high-resolution programmability, bipolar controllability, and linear/log step adjustment capability. In addition, users may observe that digital potentiometers may replace not only some of their mechanical counterparts, but also nonprecision D/A converters in some cost-sensitive applications.





Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Compact bias tees with up to 35 GHz bandwidth
31 March 2021, RF Design , Analogue, Mixed Signal, LSI
Designers can save design time and time-to-market by using a proven bias solution from the high-frequency specialist company, Marki Microwave. Its new line of surface mount bias tees, featuring up to ...

Read more...
Bidirectional logic-level voltage translators
31 March 2021, Altron Arrow , Analogue, Mixed Signal, LSI
The 74LVC2T45 and 74LVCH2T45 from Diodes Incorporated provide logic-level voltage translation between devices that operate from different supply rails. The devices have dual power supplies and support ...

Read more...
Evaluate 3D magnetic sensors in under ten minutes
31 March 2021, Altron Arrow , Analogue, Mixed Signal, LSI
You probably think that it’s not possible to evaluate a 3D magnetic sensor in less than ten minutes? Think again! Anything is possible with Infineon’s 2GO kits. Its 3D magnetic sensor 2GO kit is a budget-priced ...

Read more...
How best to calculate DAC signal chain error budget
26 February 2021 , Analogue, Mixed Signal, LSI
Electrical signal chains can be found in various forms. They can consist of different electrical components, including sensors, actuators, amplifiers, analog-to-digital converters (ADCs), digital-to-analog ...

Read more...
Inline industrial temperature sensor
26 February 2021, TRX Electronics , Analogue, Mixed Signal, LSI
Amphenol Advanced Sensors offers an inline flow-through temperature sensor that monitors the temperature of a fluid that passes through it. A system control module receives this temperature reading ...

Read more...
Multi-parameter vital signs monitoring is easier than ever before
31 March 2021, Altron Arrow , Analogue, Mixed Signal, LSI
There is rapid expansion and adoption of digital healthcare fuelled by the continuous advancement in electronics and breakthroughs in cloud computing, AI, and communication technologies.

Read more...
Closed-loop MEMS accelerometer
26 February 2021, Electrocomp , Analogue, Mixed Signal, LSI
TDK announced the release of Tronics AXO315, a miniature, 1-axis closed-loop MEMS accelerometer with a 24-bit digital SPI interface and SMD package that reaches quartz sensor performance, outperforms ...

Read more...
Shielded 1,0 GHz A/D driver
25 November 2020, RFiber Solutions , Analogue, Mixed Signal, LSI
The AM3073A from Atlanta Micro is a shielded A/D driver module that provides amplification and anti-aliasing filtering of the 1,0 GHz IF output of the AM9017 tuner module. The AM3073A offers 500 MHz ...

Read more...
Motion sensors for industrial applications
26 February 2021, Electrocomp , Analogue, Mixed Signal, LSI
TDK has launched a new compact and low-power InvenSense SmartIndustrial sensor platform family. With its support for an extended temperature range of -40°C to 105°C, the new family of 3-axis accelerometers ...

Read more...
How a 16-bit output module can be controlled with full isolation from the MCU
25 November 2020, Altron Arrow , Analogue, Mixed Signal, LSI
Whether in buildings or on production floors, programmable controllers are needed everywhere today to regulate various processes, machines, and systems. This involves programmable logic controllers (PLCs) ...

Read more...