Analog Integrated Circuits – Accurate multi-channel A/D converters

Many applications for medical, military, instrumentation, or control systems require an accurate A/D converter with at least four independent units and a moderate sampling rate. The independent A/Ds allow simultaneous sampling — but some single converter modes use a sample/hold on each channel for essentially the same thing, which is, I guess, OK too. So, for this column we will look over a few available A/Ds that have four or more converters, at least 16-bit resolution (typ), and sample at least 200,000 times per second.

First we have the AD7656 from Analog Devices with six independent 16-bit 250-ksample/s converters in a 64-LQFP package. Integral nonlinearity for this device is ±3 LSB max for the −40° to +85°C version and ±4.5 LSB max for the −40° to +125°C version.

The device can perform independent simultaneous sampling of the three input pairs and has both high-speed parallel and serial interfaces. It also has a daisy-chain feature that allows multiple A/Ds to connect to a single serial interface.

The device can accommodate true bipolar input signals in the ±4 × V_REF range and ±2 × V_REF range and it has a 2.5-V reference with ±150-ppm long-term stability and 6-ppm/°C typ temperature coefficient. The IC takes only 140 mW at 250 ksamples/s with 5-V supplies. SNR is 84 dB min, and THD is -90 dB max. It starts at $12.45 ea/1,000.

 Fig 1: The AD7656 six-channel simultaneous sampling bipolar 16-bit A/D converter.

From Maxim Integrated we have the MAX11047 quad 16-bit A/D with ±2-LSB integral nonlinearity. Each channel has a track-and-hold amp. Throughput is 250 ksamples/s for each channel, and conversion time is 3 µs.

This chips internal reference is 4.096 V ±0.39% with a temp coefficient of ±4 ppm/°C. The IC runs from 5 V over -40° to 85°C with 92-dB typ SINAD and takes about 46 mA. Shutdown current is 20 µA. It comes in a 8 x 8-mm 56-pin TQFN package, and you can also get six- and eight-channel versions.

Looking at Texas Instruments , you can find the ADS8363 dual, 16-bit, 1-Msample/s A/D with eight pseudo- or four fully differential input channels grouped into two pairs for simultaneous signal acquisition. Its claim to fame is a SNR of 93 dB and THD of -98 dB typ. INL is ±4 LSB (±1.5 typ), and CMRR is 92 dB. The device's internal 2.5-V reference is programmable and is stable to ±10 ppm/°C. The chip operates from -40° to 125°C and has a 16-mA supply current at 3.6 V. Power-down mode takes only 5 µA.

Fig. 2: The ADS8363 dual, 16-bit, 1-Msample/s A/D.

I’m going to let one outliner slip in. The Linear Technology LTC2321-16 is only a dual unit, without simultaneous sampling, but it has two excellent features. First, it takes only 31 mW per channel and offers nap and sleep modes down to 5 μW. Second, it has 2-Msample/s throughput, so it’s fast and low power.

The device's INL is ±4 LSB typ, and it yields an 81 dB SNR and –90 dB THD typ at 500-kHz input. The IC also features guaranteed operation to 125°C and it uses a single 3.3- or 5-V supply. The chip's 2.048- or 4.096-V internal reference has 20ppm/°C max stability. It uses a CMOS or LVDS SPI-compatible serial I/O and comes in a small 4 × 5-mm QFN28 package. Pricing starts at $4.95 ea/1,000.