PGA Streamlines Sensor System Design

Sensor outputs typically require some form of signal conditioning before analog-to-digital conversion. In a system with just a single sensor, an op amp with fixed resistors may be adequate. The op amp's output may be fed directly to a microcontroller (MCU), like the PICmicro, which has an on-chip analog-to-digital converter (ADC). Yet as the sensor count rises, so does design complexity. With multiple sensors in place, designers might need to provide separate gain adjustment for each sensor. Some systems may require the ability to adjust gain in the system, during setup or operation. Traditionally, designers have needed multiple components to satisfy these requirements.

The MCP6S2x, a programmable-gain amplifier (PGA) from Microchip Technology, simplifies and lowers the cost of sensor system design by integrating the necessary amplifier and control functions. Specifically, the MCP6S2x integrates a high-performance PGA with an analog multiplexer (MUX) that accepts as many as eight sensor output signals.

The MCP6S2x's on-chip logic enables designers to select the desired sensor channel and set the corresponding gain via the chip's serial-peripheral interface (SPI) port (see the figure). Gain is selectable in eight steps. There are four decimal steps (1, 2, 5, and 10 V/V) and six binary steps (1, 2, 4, 8, 16, and 32 V/V). These gain steps let the PGA autorange with the same resolution as the subsequent ADC.

A combined PGA with analog multiplexer and SPI control logic has several advantages over existing design solutions. For example, the traditional use of multiple op amps requires an ADC or MCU that can accept multiple inputs.

The MCP6S2x's single-amplifier approach slashes cost by eliminating the need for a multi-input ADC or for a more expensive MCU that handles multiple inputs. Moreover, having multiple channels conditioned on the same signal path permits self-calibration of the system, which can improve system accuracy over time and temperature.

Though it's possible to obtain a similar effect by combining a multiplexer IC with an op amp, that would require a two-chip solution. It also wouldn't address the need to adjust the gain setting for different sensors.

Op-amp gain may be made programmable by adding gain switches or a digital potentiometer. Naturally, this adds further cost and complexity. Another option is to replace the op amp with one of the existing PGAs. But many of these devices are costly chips for video applications or may target specific sensor types. Another potential disadvantage with PGAs is that they may need multiple I/O lines for gain control. This adds to the MCU's burden, unlike the MCP6S2x, which exploits the SPI bus (see "A Multisensor Design Example," p. 46).

One other important distinction for the MCP6S2x is that it doesn't experience the usual op-amp degradation in bandwidth over gain. Because the PGA has two additional bands of compensation, the bandwidth does not simply rolloff versus gain. The MCP6S2x starts with a −3-dB bandwidth of 12 MHz at a gain of 1 V/V, falls off to 6 MHz at 2V/V, rises to 10 MHz at 4 V/V, and then declines to 2 MHz again at 10 V/V and 32 V/V.

For similar performance with an op amp (one with comparable offset, noise, phase margin, etc.) the device would need a gain bandwidth product of 64 MHz. That higher specification would likely hike up the amplifier's price. Also, the op amp's stability may be harder to maintain.

Another feature is the PGA's low quiescent current. At 32-V/V gain and 2-MHz bandwidth, typical I_Q will be just 1.1 mA. That can be reduced to 1 µm maximum by placing the chip in shutdown mode via the SPI bus.

The MCP6S2x provides single-ended rail-to-rail operation from 2.7- to 5.5-V supplies from −40°C to 125°C. Low noise performance (typically 10 nV/√Hz), low offset voltage (less than 150 µV), and low gain error (under 1%) enhance system accuracy. The PGA's 200-ns settling time is fast enough to sample slow sensor signals like temperature and pressure with the effect of simultaneous sampling.

The PGA's V_REF pin offers access to the reference voltage on the internal resistor-ladder network, making the node available for active filter designs and for setting the gain at values other than those specified. This allows the MCP6S2x to achieve the high levels of gain resolution possible with other means of sensor signal conditioning.

Pricing in 1000-piece quantities ranges from $0.84 per unit for the single-channel version up to $1.76 for the eight-channel device. Samples are available now, with production units expected in April.