MP3275
APPLICATION INFORMATION
The MP3275 is a complete A/D converter system, with its
ownbuilt-inreferenceandclock. Itmaybeusedbyitself(“stand-
alone” operation), or it may be interfaced with a microprocessor.
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.
Successful application of the MP3275 requires careful atten-
tion to four main areas:
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.
1) Physical layout.
2) Connection/Trimming according to mode of operation.
3) Conditioning of input signals.
4) Control and Timing considerations.
Physical Layout
130k
1 of 16
26k
The 12-bit accuracy of the MP3275 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.
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
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Figure 7. Equivalent Input Circuit
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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.
Quasi Differential Sampling
Method 1
“Stand-Alone” Operation
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.
TheMP3275canbeusedin“stand-alone”operation, whichis
useful in systems not requiring full computer bus interface capa-
bility.
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-
trolledby WR. The3-statebufferSDOisenabledwhenRDgoes
low. There are two possible conditions that the 3-state buffer
could be in during a conversion. If RD goes low prior to WR the
output buffer is enabled and the data from the previous conver-
sion 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 buffer.
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 buffer remains 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.
Rev. 4.00
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