POWER AND LINEAR IC SOLUTIONS

High Performance SAR ADCs

When it comes to quickly digitising analogue signals from a few Hz to a few MHz, Successive Approximation Register (SAR) Analogue-to-Digital Converters (ADCs) such as those from Linear Technology are the best choice for a broad range of applications.

Fast response and low latency make SAR ADCs ideal for single- or multi-channel data acquisition. SAR ADCs offer numerous features and benefits. Low power SAR ADCs are crucial as more designs migrate to lower supply voltages and power budgets. Solution size is also a key requirement for designers needing a single snapshot of the input as many low power SAR ADCs are used in portable or multi-channel systems in which PCB space is limited. With designers trying to do more with less space, a small package becomes vital. As package size shrinks, a serial interface becomes the choice for communication. Compared to a parallel interface, serial interfaces mean far fewer data lines. This allows both the SAR ADC and the microprocessor to have smaller solution size. Linear Technology features multiple families of fast SAR ADCs for numerous applications that combine all of these features: speed, low power, small package size and simple serial interfaces.


Figure 1. The LTC2351-14 Includes Six Sample and Hold Amplifiers

One application that benefits from simultaneous sampling SAR ADCs is motor control. This application demands simultaneous sampling ADCs with multiple Sample and Hold Amplifiers (S/HAs) to measure multiple channels at the same instant and preserve the phase relationship. Data can be stored internally, to be read out sequentially, with the phase relationship from the inputs intact. Without simultaneous sampling, control algorithms could incorrectly adjust the motor’s torque or speed control, leading to vibrations and additional wear on the motor. Linear Technology has a growing family of low power, simultaneous sampling ADCs that target motor control, servos and general purpose AC power monitoring.

Linear Technology offers four six channel, simultaneous sampling ADCs optimised for two fast sample rates (250kSPS per channel and 100kSPS), as well as two different resolutions (14-bit and 12-bit). All of these low power, simultaneous sampling ADCs are pin and software compatible, making it easy to optimise the design for resolution, speed and cost. By using a 5mm x 5mm, 32-pin, QFN package, these ADCs achieve a solution size as much as six times smaller than competitive ADCs. A single 3V supply that powers both the analogue and digital circuitry reduces the power dissipation and makes it possible to do away with higher voltage supplies.

The 14-bit LTC2351-14 is a 1.5MSPS, low power SAR ADC with six simultaneously sampled differential input channels. Operating from a single 3V supply, the LTC2351-14 features six independent S/HAs and a single ADC. The single ADC with multiple S/HAs enables excellent range match (1mV) between channels and channel-to-channel skew (200ps). The six channels can monitor two separate motors providing vital information about motor torque, speed, shaft position and direction.

Table 1: Simultaneous Sampling ADCs from Linear Technology

Part No.	Resolution	Number of Channels	Sample Rate per Channel	Power	Package	Input Voltage
LTC2351-14	14	6	250kSPS	16.5mW	QFN-32 (5 mm x 5 mm)	±1.25V, 0V to 2.5V
LTC1408	14	6	100kSPS	15mW	QFN-32 (5 mm x 5 mm)	±1.25V, 0V to 2.5V
LTC1407A	14	2	1.5MSPS	14mW	MSOP-10	0V to 2.5V
LTC1407A-1	14	2	1.5MSPS	14mW	MSOP-10	±1.25V
LTC2351-12	12	6	250kSPS	16.5mW	QFN-32 (5 mm x 5 mm)	±1.25V
LTC1408-12	12	6	100kSPS	15mW	QFN-32 (5 mm x 5 mm)	±1.25V, 0V to 2.5V
LTC1407	12	2	1.5MSPS	14mW	MSOP-10	±1.25V, 0V to 2.5V
LTC1407-1	12	2	1.5MSPS	14mW	MSOP-10	±1.25V

Three input-select lines configure the number of differential inputs converted. This makes higher speeds possible as the number of channels converted decreases from six differential inputs at 250kSPS, two differential inputs at 750kSPS, to one differential input at 1.5MSPS. For lower resolution applications and performance/cost optimisation, Linear Technology offers the pin and software compatible 12-bit LTC2351-12 ADC.

Linear Technology also offers a second pair of six channel, simultaneous sampling ADCs optimised for slower sampling rates. The 14-bit LTC1408 and 12-bit LTC1408-12 are optimised for output rates up to 100kSPS/channel for all six channels.

For applications such as encoders and communications requiring simultaneous sampling on only two channels at rates greater than 1MSPS per channel, Linear Technology offers a pin and software compatible LTC1407 family of 14-bit and 12-bit, 2 channel, simultaneous sampling SAR ADCs. See Table 1.


Fast Single Channel SAR ADCs
Along with its growing family of simultaneous sampling ADCs, Linear Technology is also adding to its family of pin and software compatible high speed, single channel SAR ADCs. The 14-bit, 3.5MSPS LTC2356-14 measures a single differential input and communicates via an SPI-compatible serial interface. This SAR ADC operates from a single 3.3V supply, draws only 18mW at the maximum conversion rate and is available in a tiny 10-pin MSOP package. The combination of high speed, low power and small package makes the LTC2356-14 ideal for high speed, portable applications including data acquisition, communications and medical instrumentation.

Table 2: Fast Single-Channel SAR ADCs from Linear Technology

Part Number	Resolution	Sample Rate	Package	Power	Input Voltage Range	I/O
LTC2355-14	14	3.5MSPS	MSOP-10	18mW	0V to 2.5V	Serial
LTC2356-14	14	3.5MSPS	MSOP-10	18mW	±1.25V	Serial
LTC1403A	14	2.8MSPS	MSOP-10	14mW	0V to 2.5V	Serial
LTC1403A-1	14	2.8MSPS	MSOP-10	14mW	±1.25V	Serial
LTC2355-12	12	3.5MSPS	MSOP-10	18mW	0V to 2.5V	Serial
LTC2356-12	12	3.5MSPS	MSOP-10	18mW	±1.25V	Serial
LTC1403	12	2.8MSPS	MSOP-10	14mW	0V to 2.5V	Serial
LTC1403-1	12	2.8MSPS	MSOP-10	14mW	±1.25V	Serial

Summary
With PCB real estate getting tighter and designers continually searching for lower power ICs, fast data acquisition can be a challenge. Price can often be another obstacle with cost conscious designers or managers looking to trim their total system costs. Linear Technology’s families of simultaneous sampling ADCs and fast single channel ADCs make it possible to optimise solution size, power and cost. The pin and software compatible families of six channel, two channel and single channel ADCs offer flexibility to upgrade from 12-bit resolution to 14-bit resolution. Whatever the motor control, power monitoring or data acquisition system requires, Linear Technology has a fast SAR ADC to do the job.

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