AD7870A
V(i) the estimated code transition point is derived as follows:
noisy code transitions. Other causes of concern are ground loops
and digital feedthrough from microprocessors. These are factors
that influence any ADC, and a proper PCB layout that mini-
mizes these effects is essential for best performance.
π ×cum(i )
V(i )= – A×Cos
N
where A is the peak signal amplitude,
N is the number of histogram samples
and cum(i) = ∑in = 0 V(n) occurrences
LAYOUT HINTS
Ensure that the layout for the printed circuit board has the digi-
tal and analog signal lines separated as much as possible. Take
care not to run any digital track alongside an analog signal track.
Guard (screen) the analog input with AGND.
Establish a single point analog ground (star ground) separate
from the logic system ground at the AD7870A AGND pin or as
close as possible to the AD7870A. Connect all other grounds
and the AD7870A DGND to this single analog ground point.
Do not connect any other digital grounds to this analog ground
point.
Low impedance analog and digital power supply common re-
turns are essential to low noise operation of the ADC, so make
the foil width for these tracks as wide as possible. The use of
ground planes minimizes impedance paths and also guards the
analog circuitry from digital noise.
Figure 14. AC INL Plot
NOISE
APPLICATION HINTS
Keep the input signal leads to VIN and signal return leads from
AGND as short as possible to minimize input noise coupling. In
applications where this is not possible, use a shielded cable be-
tween the source and the ADC. Reduce the ground circuit im-
pedance as much as possible since any potential difference in
grounds between the signal source and the ADC appears as an
error voltage in series with the input signal.
Good printed circuit board (PCB) layout is as important as the
overall circuit design itself in achieving high speed A/D perfor-
mance. The AD7870A is required to make bit decisions on an
LSB size of 1.465 mV. Thus, the designer has to be conscious of
noise both in the ADC itself and in the preceding analog circuitry.
Switching mode power supplies are not recommended as the
switching spikes will feed through to the comparator causing
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
24-Pin Plastic DIP (N-24)
REV. 0
–11–
ADI [ ADI ]
