MP3276
APPLICATION INFORMATION
The MP3276 is a complete A/D converter system, with its
ownbuilt-inreferenceandclock. Itmaybeusedbyitself(“stand-
alone” operation), or it may be interfaced with a microprocessor
which can control both conversion and formatting of output.
imperative that RD or WR not change during a conversion to in-
sure that errors will not occur.
Ground Reference
Successful application of the MP3276 requires careful atten-
tion to four main areas:
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.
1) Physical layout.
2) Connection/Trimming according to mode of operation.
3) Conditioning of input signals.
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.
4) 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.
130k
1 of 16
26k
COMP
•
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.
GND Ref.
130k
S
A
R
1/2
V
26k
REF
VDAC
12
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Power supplies should be quiet and well regulated.
Grounds should be tied together at the package and
backtothesystemgroundwithasinglepath. Bypassthe
supplies at the device with a 0.01 to 0.1µF ceramic cap
and a 10-47 µF tantalum type, in parallel.
Figure 7. Equivalent Input Circuit
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.
“Stand-Alone” Operation
TheMP3276canbeusedin“stand-alone”operation, whichis
useful in systems not requiring full computer bus interface capa-
bility. Thisoperationisavailableforeitherparallelorserialmode.
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.
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.
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.
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
Rev. 4.00
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