LTC1090CJ View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1090CJ' PDF : 28 Pages View PDF
LTC1090
APPLICATIO S I FOR ATIO
3. Analog Inputs
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1090 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 LTC1090 look like a 60pF capaci-
tor (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.
VIN+
RSOURCE+
“+”
INPUT
C1
VIN–
RSOURCE–
“–”
INPUT
C2
4TH SCLK
RON = 500Ω
LAST SCLK
LTC1090
CIN = 60pF
LTC1090 • AI16
Figure 9. Analog Input Equivalent Circuit
“+” Input Settling
This input capacitor is switched onto the “+” input during
the sample phase (tSMPL, see Figure 10). The sample
phase starts at the 4th SCLK cycle and lasts until the falling
edge of the last SCLK (the 8th, 10th, 12th or 16th SCLK
cycle depending on the selected word length). The voltage
on the “+” input must settle completely within this sample
time. Minimizing RSOURCE+ and C1 will improve the input
settling time. If large “+” input source resistance must be
used, the sample time can be increased by using a slower
SCLK frequency or selecting a longer word length. With
the minimum possible sample time of 4µs, RSOURCE+ < 2k
and C1 < 20pF will provide adequate settling.
“–” Input Settling
At the end of the sample phase the input capacitor switches
to the “–” input and the conversion starts (see Figure 10).
During the conversion, the “+” input voltage is effectively
“held” by the sample and hold and will not affect the
conversion result. However, it is critical that the “–” input
voltage be free of noise and settle completely during the
first four ACLK cycles of the conversion time. Minimizing
RSOURCE– and C2 will improve settling time. If large
“–” input source resistance must be used, the time allowed
for settling can be extended by using a slower ACLK
frequency. At the maximum ACLK rate of 2MHz, RSOURCE–
< 1kΩ and C2 < 20pF will provide adequate settling.
CS
SCLK
ACLK
“ + ” INPUT
“ – ” INPUT
MUX ADDRESS
SHIFTED IN
SAMPLE “ + ” INPUT MUST
HOLD
SETTLE DURING THIS TIME
t SMPL
1
2
3
4
LAST SCLK (8TH, 10TH, 12TH OR 16TH DEPENDING ON WORK LENGTH)
1234
1ST BIT
TEST
“ – ” INPUT MUST SETTLE
DURING THIS TIME
Figure 10. “+” and “–” Input Settling Windows
LTC1090 • AI17
1090fc
19
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