V– 22 F TANTALUM VCC LTC1290 F06 0.1 F" />
19
LTC1290
1290fe
V–
22
F
TANTALUM
VCC
LTC1290 F06
0.1
F
CERAMIC
DISK
ANALOG
GROUND
PLANE
1
10
20
U
S
A
O
PPLICATI
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, VCC must be free of noise and
ripple. Any changes in the VCC voltage with respect to
analog ground during a conversion cycle can induce
errors or noise in the output code. VCC noise and ripple can
be kept below 0.5mV by bypassing the VCC pin directly to
the analog ground plane with a 22
F tantalum capacitor
and leads as short as possible. The lead from the device to
the VCC supply should also be kept to a minimum and the
VCC 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 VCC
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 VCC Bypassing.
Noise and Ripple Can Cause A/D Errors
VERTICAL:
0.5mV/DIV
HORIZONTAL: 10
s/DIV
CS
VCC
Figure 8. Good VCC Bypassing Keeps
Noise and Ripple on VCC Below 1mV
HORIZONTAL: 10
s/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 (CIN) in series with a 500 resistor (RON) as
shown in Figure 9. CIN 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.
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