Datasheet

19
LTC1290
1290fe
V
–
22µF
TANTALUM
V
CC
LTC1290 F06
0.1µF
CERAMIC
DISK
ANALOG
GROUND
PLANE
1
10
20
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Figure 6. Example Ground Plane for the LTC1290
Figure 6 shows an example of an ideal ground plane design
for a two-sided board. Of course, this much ground plane
will not always be possible, but users should strive to get
as close to this ideal as possible.
2. Bypassing
For good performance, V
CC
must be free of noise and
ripple. Any changes in the V
CC
voltage with respect to
analog ground during a conversion cycle can induce
errors or noise in the output code. V
CC
noise and ripple can
be kept below 0.5mV by bypassing the V
CC
pin directly to
the analog ground plane with a 22µF tantalum capacitor
and leads as short as possible. The lead from the device to
the V
CC
supply should also be kept to a minimum and the
V
CC
supply should have a low output impedance such as
that obtained from a voltage regulator (e.g., LT1761).
Figures 7 and 8 show the effects of good and poor V
CC
bypassing.
3. Analog Inputs
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1290 have
capacitive switching input current spikes. These current
Figure 7. Poor V
CC
Bypassing.
Noise and Ripple Can Cause A/D Errors
VERTICAL: 0.5mV/DIV
HORIZONTAL: 10µs/DIV
CS
V
CC
Figure 8. Good V
CC
Bypassing Keeps
Noise and Ripple on V
CC
Below 1mV
HORIZONTAL: 10µs/DIV
VERTICAL: 0.5mV/DIV
spikes settle quickly and do not cause a problem. How-
ever, if large source resistances are used or if slow settling
op amps drive the inputs, care must be taken to insure that
the transients caused by the current spikes settle com-
pletely before the conversion begins.
Source Resistance
The analog inputs of the LTC1290 look like a 100pF
capacitor (C
IN
) in series with a 500Ω resistor (R
ON
) as
shown in Figure 9.
C
IN
gets switched between the selected
“+” and “–” inputs once during each conversion cycle. Large
external source resistors and capacitances will slow the
settling of the inputs. It is important that the overall RC time
constants be short enough to allow the analog inputs to
completely settle within the allowed time.