
17
LTC1283
1283fb
U
S
A
O
PPLICATI
WU
U
I FOR ATIO
or noise in the output code. VCC noise and ripple can be
kept below 1mV by bypassing the VCC pin directly to the
analog ground plane with a 4.7
μF tantalum with leads as
short as possible. 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 LTC1283 have
capacitive switching input current spikes. These current
spikes settle quickly and do not cause a problem.
However, 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 completely before the conversion begins.
Source Resistance
The analog inputs of the LTC1283 look like 65pF capacitor
(CIN) in series with a 500Ω 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.
Figure 6. Example Ground Plane for the LTC1283
V–
4.7
μF
TANTALUM
VCC
LTC1283 F06
0.1F
CERAMIC
DISK
ANALOG
GROUND
PLANE
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
VERTICAL:
0.5mV/DIV
HORIZONTAL: 10
μs/DIV
Figure 7. Poor VCC Bypassing. Noise and Ripple
can Cause A/D Errors
VERTICAL:
0.5mV/DIV
HORIZONTAL: 10
μs/DIV
Figure 8. Good VCC Bypassing Keeps Noise and Ripple
on VCC Below 1mV
4TH SCLK
RON = 500Ω
LAST SCLK
CIN =
65pF
LTC1283
“+”
INPUT
RSOURCE
+
VIN
+
C1
“–”
INPUT
RSOURCE
–
VIN
–
C2
LTC1283 F09
Figure 9. Analog Input Equivalent Circuit