22-bit A/Ds reside in 8-pin MSOP

Chandler, Ariz. — Microchip Technology Inc. claims these 22-bit analog-to-digital (A/D) converters are among the lowest in power consumption and among the smallest package-sized high-resolution A/D converters in the industry.

Microchip's MCP3551 and MCP3553 delta-sigma, A/D converters, offered in a 8-pin MSOP (3.1x 3.1mm), and with typical power consumption of 120 uA, are aimed at consumer (weigh scales and handheld meters), industrial (instrumentation, pressure sensors, weigh scales, handheld meters and multimeters), medical (heart rate monitors and blood glucose meters), automotive (sensor interface), portable and battery-powered applications.

In addition, these devices offer integral non-linearity (INL) of ±2 ppm typical, power consumption of 0.85 mW maximum at 5 V, and output noise as low as 2.5 uV rms. They also provide auto calibration with every conversion, and an extended temperature range of -40 °C to +125°C.

The MCP3551 has a sample rate of 13.75 samples/s and an effective resolution of 21.9 bits, while the MCP3553 has a sample rate of 60 samples/s and an effective resolution of 20.6 bits.

The MCP3551 and MCP3553 are available today. Microchip is offering the MCP3551 PICtail demonstration board (MCP3551DM-PCTL) for $35.00.

See related block diagram

Pricing for both the MCP3551 and MCP3553 is $2.67 in the 8-pin MSOP package and $2.64 in the 8-pin SOIC package in 10,000-piece quantities. Click here for the MCP3551 and MCP3553 data sheets.

Microchip , 1-888-628-6247,

These single-channel A/D converters are Microchip's first high-resolution converters utilizing the delta-sigma architecture. Although Microchip is known mostly for their microcontrollers, the company has many linear, thermal management, power management, mixed-signal and interface products.

Microchip has been integrating analog ICs into their MCUs for over a decade. About a decade ago, they began offering stand-alone analog and interface products. Microchip's acquisition of TelCom Semiconductor Inc. (Mountain View, Calif.) in 2001 beefed up its stand-alone analog IC offering significantly, said Trent Butcher, senior product marketing engineer with Microchip's Analog and Interface Products Division.

Click here for more details on the acquisition.

Prior to this announcement, Microchip's A/D converter portfolio consisted of 16- or 17-bit, dual-slope devices, Butcher said.

The main difference between the two devices that Microchip is rolling out today is that the MCP3553 samples at a faster rate (60 samples/s), versus the MCP3551 (13.75 samples/s). The tradeoff for using a faster device is that you lose some of the effective bit resolution (about 1 ½ bits due to noise), Butcher said. “Resolution goes from 21.9 in the MCP3551 down to 20.6 in the MCP3553,” he said.

Many 24-bit A/D converters typically provide only 20 bits of effective resolution because of output noise, but Microchip's A/D converters give you the same number of effective bits or more, according to Butcher. “The noise in these devices is so low that you almost get the full 22-bits of resolution. On the MCP3551, you get 21.9 ENOB typically,” he said.

Microchip's high ENOB is related directly to its output noise specification (2.5 uV rms), as you can see in the graph below. It shows minimal output noise with changes in input voltage.

Output Noise vs. Input Voltage graph

To compensate for additional noise, Microchip has incorporated a notch filter in the MCP3551. The converter's 50/60 Hz rejection feature is application generic for all geographic regions. The line frequency is 60 Hz in the United States and 50 Hz in Europe and Asia — so the part can be used in different parts of the world. Notch filters are commonly used in devices that sample at less than 20 to 30 samples/s. Without a notch filter, there is more noise from the line frequency coming through the power supply and, hence, lower overall resolution.

Although A/D converters with similar INL are not uncommon, they do not provide this level of INL at this price, Butcher said. “There are devices with similar INL, but they will cost twice as much. Generally, the maximum INL for comparable devices is 10 to 20 ppm, versus 6 ppm max for the MCP3551 and MCP3553,” he said.

Low power is another key attribute of the A/D converters. The MCP3551 consumes 120 uA at 5 V — and about 100 uA if you go down to 2.7 V. The MCP3553 operates at a higher current, so it uses more power (140 uA at 5 V). “At the 20-24 bit resolution level, at this same sample rate, most A/D converters will typically consume 300 uA,” Butcher said.

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