Six-channel A/D does simultaneous sampling

Milpitas, Calif. — Linear Technology Corp.'s 14-bit, 600 Ksamples per second analog-to-digital (A/D) converter with six simultaneous sampling differential inputs draws 5 milliamps from a single 3-volt supply.

This 3-wire serial A/D converter offers typical power dissipation of 15 milliwatts. The LTC1408 is available in a 32-pin (5 x 5-mm) QFN package. With less than 1/10th the power consumption and 1/5th the package size of the nearest competitor, the LTC1408 allows the design of compact, battery-powered, and portable data acquisition systems, the company said. The six simultaneously sampled differential inputs make this device practical for multiphase power measurement, multiphase motor control, data acquisition systems, and uninterruptible power supplies.

When the LTC1408 A/D converter is not converting, power dissipation can be further reduced to 3.3 milliwatts in nap mode with the internal 2.5-volt reference remaining active, and to 6 microwatts in sleep mode, with all internal circuitry powered down. The internal reference can also be overdriven with an external reference, up to the analog supply voltage.

The LTC1408 converter uses three input-select lines to configure the number of differential inputs converted. Thus, higher speeds are possible, from one differential input at 600 Ksamples/s to six differential inputs at 100 Ksamples/s. The six conversion results are delivered sequentially to a high speed digital signal processing (DSP) serial port via a 3-wire interface. This A/D converter also features a separate digital output power supply pin and a bipolar/unipolar pin to select ±1.25-V bipolar or 0 V to 2.5 V unipolar input ranges.

See related block diagram

The LTC1408 A/D converter is available in the commercial and industrial temperature ranges. Pricing begins at $8.95 in 1,000-piece quantities. Click here for the LTC1408 data sheet.

Linear Technology Corp. , 1-800-454-6327,

Simultaneous sampling is what makes these A/D converters stand out. There are only a small number of A/D converters that can do simultaneous sampling. “This is a dedicated architecture for the application because it requires a lot more circuitry,” said Todd Nelson, product marketing manager of LTC's mixed-signal products.

Previously, LTC provided A/D converters with a lot of multiplexed input channels. There's a big difference between multiplexed inputs and simultaneous sampling. With multiplexed input parts, for example, there is a single A/D converter with a single sample and hold (S/H) amplifier, and a multiplexer in front of it to select the number of channels needed. “That way, channels are measured sequentially — one at a time and they are all delayed slightly in time. This is fine for most applications — but when the phase relationship of multiple channels is important, the time delay can cause system-level problems,” Nelson said.

With the LTC1408 A/D converter, however, there are six S/Hs in the circuit so the converter will grab the sample at each input and hold it. “This way you can sequentially convert each channel — and the output will reflect what the signals were all at the same time. The six S/Hs capture the six signals at the exact time. Then, the conversion takes place sequentially but the data can be compared directly because it was sampled with no time delay,” Nelson explained.

LTC made a similar part a few years ago with two channels of simultaneous sampling. The LTC1407 targeted different applications though. “Its much higher speed (3 Msamples/s) lends itself to narrow-band applications like communications,” Nelson said.

Some applications require simultaneous sampling. Either multiple A/D converters or multiple external S/H amplifiers without multiplexers were used to achieve the same result previously, according to Nelson. Sampling multiple inputs at the same time to preserve a phase relationship such as a three-phase power measurement or motor control application where you measure voltage and current are two such applications that require simultaneous sampling. “In motor control, voltage and current need to be measured simultaneously because algorithms won't work with multiplexed parts,” Nelson said. “The errors caused by the phase delay or the errors in time can cause instability in the control loop — leading to excess vibration and wear if not more serious problems,” he added.

LTC isn't saying that this is the only six-channel A/D converter around. LTC is addressing the need for smaller, lower power six-channel A/D converters with the LTC1408, Nelson said. “Typical parts out there that perform similar functions are five times bigger and consume three times more power,” he said. The device, packaged in a 32-pin, 5 x 5-mm QFN, draws only 5 mA from a single 3-V supply.

Another unique aspect of the LTC part is that it's more flexible than it appears. On the surface, it looks like a six-channel A/D converter. If, however, only four channels are needed, the part can be configured for only four channels — without sacrificing any speed, Nelson said. “You could measure voltage and current on two signals instead of three, for instance. Usually, a part like this wouldn't be flexible. It would be a fixed function to simultaneously sample a specific number of channels,” he said.

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