LTC1096IS8 Linear Technology, LTC1096IS8 Datasheet - Page 23

• Manufacturer Part Number: LTC1096IS8
• Description: IC A/D CONV 8BIT SRL IN/OUT8SOIC
• Manufacturer: Linear Technology

Specifications of LTC1096IS8

• Number Of Bits: 8
• Sampling Rate (per Second): 33k
• Data Interface: Serial
• Number Of Converters: 1
• Power Dissipation (max): 600µW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS INFORMATION
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.
RC Input Filtering
It is possible to fi lter the inputs with an RC network as
shown in Figure 9. For large values of C
capacitive input switching currents are averaged into a
net DC current. Therefore, a fi lter should be chosen with
a small resistor and large capacitor to prevent DC drops
across the resistor. The magnitude of the DC current is
approximately I
portional to V
of 29μs, the input current equals 4.3μA at V
case, a fi lter resistor of 390Ω will cause 0.1LSB of full-
scale error. If a larger fi lter resistor must be used, errors
can be eliminated by increasing the cycle time.
Input Leakage Current
Input leakage currents can also create errors if the source
resistance gets too large. For instance, the maximum input
leakage specifi cation of 1μA (at 125°C) fl owing through a
source resistance of 3.9k will cause a voltage drop of 3.9mV
or 0.2LSB. This error will be much reduced at lower tem-
peratures because leakage drops rapidly (see typical curve
of Input Channel Leakage Current vs Temperature).
REFERENCE INPUTS
The voltage on the reference input of the LTC1096 defi nes
the voltage span of the A/D converter. The reference
input transient capacitive switching currents due to the
switched-capacitor conversion technique (see Figure 10).
During each bit test of the conversion (every CLK cycle), a
IN
V
IN
. When running at the minimum cycle time
DC
Figure 9. RC Input Filtering
R
FILTER
= 25pF(V
I
DC
C
FILTER
IN
/t
CYC
“+”
“–”
) and is roughly pro-
LTC1098
10968 F09
F
(e.g., 1μF), the
IN
= 5V. In this
capacitive current spike will be generated on the reference
pin by the ADC. These current spikes settle quickly and do
not cause a problem.
Using a slower CLK will allow more time for the reference
to settle. Even at the maximum CLK rate of 500kHz most
references and op amps can be made to settle within the
2μs bit time.
Reduced Reference Operation
The minimum reference voltage of the LTC1098 is limited
to 3V because the V
tied together. However, the LTC1096 can operate with
reference voltages below 1V.
The effective resolution of the LTC1096 can be increased
by reducing the input span of the converter. The LTC1096
exhibits good linearity and gain over a wide range of ref-
erence voltages (see typical curves of Linearity and Full
Scale Error vs Reference Voltage). However, care must be
taken when operating at low values of V
reduced LSB step size and the resulting higher accuracy
requirement placed on the converter. The following factors
must be considered when operating at low V
1. Offset
2. Noise
3. Conversion speed (CLK frequency)
Offset with Reduced V
The offset of the LTC1096 has a larger effect on the output
code when the ADC is operated with reduced reference
voltage. The offset (which is typically a fi xed voltage)
becomes a larger fraction of an LSB as the size of the
LSB is reduced. The typical curve of Unadjusted Offset
Error vs Reference Voltage shows how offset in LSBs is
Figure 10. Reference Input Equivalent Circuit
R
V
OUT
REF
REF
GND
LTC1096/LTC1096L
LTC1098/LTC1098L
5
4
+
CC
supply and reference are internally
REF
EVERY CLK CYCLE
R
ON
REF
5pF TO 30pF
because of the
LTC1096
REF
10968 F10
values.
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10968fc