MP3276AG datasheet(11/16 Pages) EXAR

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MP3276

Rev. 4.00

APPLICATION INFORMATION

The MP3276 is a complete A/D converter system, with its

own built-in reference and clock. It may be used by itself (“stand-

alone” operation), or it may be interfaced with a microprocessor

which can control both conversion and formatting of output.

Successful application of the MP3276 requires careful atten-

tion to four main areas:

Physical layout.

Connection/Trimming according to mode of operation.

Conditioning of input signals.

Control and Timing considerations.

Physical Layout

The 12-bit accuracy of the MP3276 represents a dynamic

range of 72dB. Precautions must be taken to avoid any interfer-

ing signals, whether conducted or radiated, to assure that this is

not degraded.

• Avoid placing the chip and its analog signals near logic

traces. In general, using a double sided printed circuit

card with a good ground plane on the component side is

recommended. Routing analog signals between ground

traces will help isolate digital control logic. If these lines

cross, do so at right angles. The GND Ref. is the positive

terminal of the MUX/Instrumentation amplifier and will

provide common mode noise rejection. It should be

close to and shielded together with the channel inputs in

order to take advantage of this feature.

• Power supplies should be quiet and well regulated.

Grounds should be tied together at the package and

back to the system ground with a single path. Bypass the

supplies at the device with a 0.01 to 0.1

µF ceramic cap

and a 10-47

µF tantalum type, in parallel.

“Stand-Alone” Operation

The MP3276 can be used in “stand-alone” operation, which is

useful in systems not requiring full computer bus interface capa-

bility. This operation is available for either parallel or serial mode.

For this operation, CS = 0, ADEN = 1, and conversion is con-

trolled by WR. The 3-state buffers are enabled when RD goes

low. There are two possible conditions that the 3-state buffers

could be in during a conversion. If RD goes low prior to WR, the

output buffers are enabled and the data from the previous con-

version is available at the outputs during STL = 1. At the end of

the present conversion which is initiated at the rising edge of

WR, STS returns low and the new conversion result is placed on

the output data buffers.

If WR goes low prior to RD, the data buffers remain in a high

impedance state and conversion is initiated at the rising edge of

WR. Upon the end of the conversion the STS returns low and

the conversion result is placed on the output data buffers. It is

imperative that RD or WR not change during a conversion to in-

sure that errors will not occur.

Ground Reference

The ground reference pin can be used for remote ground

sensing of a common mode input signal with a maximum 6 V p-p

around AGND.

This common input can also be used to dither each input’s

“zero”. By averaging multiple conversions digitally, higher reso-

lution for each input conversion can be obtained. Patterns for

this dither can be a ramp, a stair step, or white noise.

COMP

VDAC

130k

26k

130k

26k

1 of 16

GND Ref.

Figure 7. Equivalent Input Circuit

1/2

VREF

Quasi Differential Sampling

Method 1

For remote ground sensing where the remote ground does

not change more than

3 V from the A/D ground, connect GND

Ref to the remote ground.

Method 2

Where Method 1 applies to each channel or group of chan-

nels, add a mux to allow connecting the appropriate ground to

GND Ref.

Method 3

Use two parts. Tie both GND Ref pins together and connect

this node to the “common” remote GND. Control the sample

point by connecting each STL through an “OR” gate whose out-

put is “NAND” connect with WR (inverted WR). Use this output

as WR to both WR inputs. By controlling the WR, sample delay

differences between the two converters is minimized. Two parts

from the same date code will further minimize this difference.

Treat one A/D as the (+) terminal and the other as the (–) termi-

nal of the differential signal. Now the difference can be taken

digitally.

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