Size is certainly important in today’s electronics environment; the smaller package the better, provided the part can still be handled by smart, but standard, machines on the equipment production/assembly line. The Burr-Brown ADS 8320 is a full 16-bit ADC in the tiny 8-pin MSOP. It leapfrogs the competition on the package size front and in many of the performance specifications it achieves.

Operation of the part is over the supply range of 2.7 to 5 V and it is a pin-compatible replacement for the 12-bit ADS7822 so that a designer can drop the part in to obtain the increase in resolution and performance with few, if any, external circuit changes. Performance has been optimized for 2.7-V operation and the next part from the same stable will be a version slightly modified for optimal performance at the 5-V end of the supply range.

The part uses a non-exotic 0.6-micron CMOS process but where it differs from previous products from the same vendor — and from other sources — is in the unique architecture of how the part is powered. Basically the power is turned on to an area of operation only when it is required. This really aggressive approach to biasing provides a typical power dissipation of 1.8 mW with a 2.7-V supply (and 100-kHz sampling), which falls to 0.8 m W (0.3 m A) during power down.

Architecture

The ADS8320 (see Fig. 1) is a successive-approximation register (SAR) ADC which provides a sample-and-hold function with the distributed capacitance. The input is differential — as is the signal path through the last latch — but it is intended to only reject small signal-ground differences at the input that might exist between the remote sensing position and the local ground; that potential is limited to a maximum range of -100 mV to +500 mV when the supply is 2.7 V (-100 mV to +1 V for a 5-V supply) for a common rejection from both input lines. The full-scan input range is 0 to V_REF and the reference (external) can be from 0.5 V up to the supply voltage.

If the input signals cannot be maintained within the limits of the differential range the linearity of the part will suffer. Input current is taken only during sampling and must be sufficient to charge the 45 pF input capacitance within 4.5 clock cycles (generally 4.5 x 410 ns = 1.85 m s) and after going into the hold mode (or during power-down) the input impedance exceeds 1 GW and the leakage is less than 1 nA.

The SAR of the converter is essentially all capacitive and the linearity performance is extremely good. With about the maximum input of 2.5 V on a rail of 2.7 V, a 1-kHz input signal suffers a total harmonic distortion of -86 dB, SINAD of 84 dB and spurious-free dynamic range (SFDR) of 86 dB. The signal-to-noise ratio (SNR) is 88 dB. The latter improves to 90 dB with the rail at 5 V and with a 5-V input, but it is clear that Burr-Brown’s numbers are not accurate because of the testing scenario they are using and will improve, probably by 3-4 dB, when they get it right on the bench. Noise performance will, of course, worsen when the drive signal levels that you provide drop below these optimal levels and it is critical in such designs that the supply rail be absolutely clean to achieve the maximum numbers possible.

The serial output D_OUT (see Fig. 1, again) is MSB first in straight binary and will swing from 0 to the supply rail. (It might be important to note in some applications that if 5-V CMOS logic is driven from the ADS8320 then the logic may dissipate more current and extend the propagation time.) The digital inputs can be at levels up to 5.5 V, even if the ADC supply rail is 2.7 V.

The performance of the IC is guaranteed for clock frequencies that exceed 24-times the sampling speed with 4.5 clocks taken for the sampling, 16 clocks for the conversion, and a couple of clocks for the power-down (plus margin.) So, with sampling at 100 kHz the clock would be 2.4 MHz and this will also optimize the power dissipation to the lowest levels.

System Operation

A basic data acquisition system (see Fig. 2) where the reference is connected directly to the supply would have an input range of 0 to V_CC. The 0.1 m F ceramic bypass capacitor (particularly looking to clean spikes occurring just before the comparator latches) would be placed as close as possible to the ADS8320 package and the low-pass filter formed by the 1 - 10 m F capacitor and the 5 W resistor can reduce supply noise, and the noise produced on the supply by the microcontroller. The ground used should be analog ground and not a microcontroller ground.

Although the 16-bit nature of the ADS8320 would allow designers who only need 12-bit, say, performance to be a little lackadaisical about the input signal level, optimum performance will only be achieved if the signals are optimized for the part; to that end a designer should consider the use of a input buffer to drive the ADC at 2.5 V for a 2.7-V supply or 5 V for a 5-V supply.

Perspective

The ADS8320 is an elegantly-designed 16-bit ADC with excellent performance, particularly on the noise perspective. The power dissipation is incredibly low because of a patent-applied-for architecture of shut-downs when circuits are not in use. These things, combined with an extremely small package, have allowed Burr-Brown to produce a winner for its stable putting it probably two steps ahead of the competition today.

Although any number of parts will be designed into systems to optimize 12-bit designs that are marginal there seem to be numerous applications to really use all 16 bits to good purpose. Expect to see major design wins for the part from, particularly, remote sensor systems, multi-channel systems and robotics. Medical instrumentation is increasingly looking for more and more bits, for more and more channels, for less and less power, and in smaller and smaller sizes: the ADS8320 fits the bill.

Burr-Brown has created a real bridge part between the 12- and 16-bit worlds with a clean design with good offset and gain specifications over temperature and without trimming during production.

The ADS8320 is in production and is priced at $6.25 in 1000-piece lots. Burr-Brown is at PO Box 11400, Tucson, AZ 85734. +1 (520) 746-1111, Fax +1 (520) 889-1510. Product information (USA) (800) 548-6132.