17
®
ADS7807
The resistive front end of the ADS7807 also provides a
guaranteed
±
25V overvoltage protection. In most cases, this
eliminates the need for external over voltage protection
circuitry.
INTERMEDIATE LATCHES
The ADS7807 does have tri-state outputs for the parallel
port, but intermediate latches should be used if the bus will
be active during conversions. If the bus is not active during
conversion, the tri-state outputs can be used to isolate the
A/D from other peripherals on the same bus.
Intermediate latches are beneficial on any monolithic A/D
converter. The ADS7807 has an internal LSB size of 38
µ
V.
Transients from fast switching signals on the parallel port,
even when the A/D is tri-stated, can be coupled through the
substrate to the analog circuitry causing degradation of
converter performance.
APPLICATIONS INFORMATION
TRANSITION NOISE
Apply a DC input to the ADS7807 and initiate 1000 conver-
sions. The digital output of the converter will vary in output
codes due to the internal noise of the ADS7807. This is true
for all 16-bit SAR converters. The transition noise specifica-
tion found in the electrical specifications section is a statis-
tical figure which represents the one sigma limit or rms
value of these output codes.
Using a histogram to plot the output codes, the distribution
should appear bell-shaped with the peak of the bell curve
representing the nominal output code for the input voltage
value. The
±
1
σ
,
±
2
σ
, and
±
3
σ
distributions will represent
68.3%, 95.5%, and 99.7% of all codes. Multiplying TN by
6 will yield the
±
3
σ
distribution or 99.7% of all codes.
Statistically, up to 3 codes could fall outside the 5 code
distribution when executing 1000 conversions. The ADS7807
has a TN of 0.8 LSBs which yields 5 output codes for a
±
3
σ
distribution. See Figures 12 and 13 for 1000 and 10,000
conversion histogram results.
AVERAGING
The noise of the converter can be compensated by averaging
the digital codes. By averaging conversion results, transition
noise will be reduced by a factor of 1/
√
n where n is the
number of averages. For example, averaging four conver-
sion results will reduce the TN by 1/2 to 0.4 LSBs. Averag-
ing should only be used for input signals with frequencies
near DC.
For AC signals, a digital filter can be used to lowpass filter
and decimate the output codes. This works in a similar
manner to averaging: for every decimation by two, the
signal-to-noise ratio will improve 3dB.
QSPI INTERFACING
Figure 14 shows a simple interface between the ADS7807
and any QSPI equipped microcontroller. This interface as-
FIGURE 13. Histogram of 10,000 Conversions with Input
Grounded.
438
FFFEH
1681
FFFFH
5671
0000H
176
0001H
2010
182
0002H
0
18
0003H
FFFDH
FIGURE 12. Histogram of 1000 Conversions with Input
Grounded.
52
FFFEH
173
FFFFH
581
0000H
176
0001H
18
0002H
0
0
0003H
FFFDH
sumes that the convert pulse does not originate from the
microcontroller and that the ADS7807 is the only serial
peripheral.
Before enabling the QSPI interface, the microcontroller
must be configured to monitor the slave select line. When a
transition from LOW to HIGH occurs on Slave Select (SS)
from BUSY (indicating the end of the current conversion),
the port can be enabled. If this is not done, the microcontroller
and the A/D may be “out-of-sync”.
