LTC1283CN View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1283CN' PDF : 24 Pages View PDF
APPLICATI
S I FOR ATIO
VCC
4.7µF
TANTALUM
ANALOG
GROUND
PLANE
1
20
2
19
3
18
4
17
5
16
6
15
7
14
8
13
9
12
10
11
V–
0.1µF
CERAMIC
DISK
LTC1283 • F06
Figure 6. Example Ground Plane for the LTC1283
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 4.7µF 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.
LTC1283
HORIZONTAL: 10µs/DIV
Figure 7. Poor VCC Bypassing. Noise and Ripple
can Cause A/D Errors
HORIZONTAL: 10µs/DIV
Figure 8. Good VCC Bypassing Keeps Noise and Ripple
on VCC Below 1mV
VIN +
RSOURCE +
VIN –
RSOURCE –
“+”
INPUT
C1
“–”
INPUT
C2
LTC1283
4TH SCLK
RON = 500Ω
LAST SCLK
CIN =
65pF
LTC1283 • F09
Figure 9. Analog Input Equivalent Circuit
17
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