LTC1096 Linear Technology, LTC1096 Datasheet - Page 20

• Manufacturer Part Number: LTC1096
• Description: Micropower Sampling 8-Bit Serial I/O A/D Converters
• Manufacturer: Linear Technology

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.
RC Input Filtering
It is possible to filter 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 filter 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 29 s, the input current equals 4.3 A at V
case, a filter resistor of 390 will cause 0.1LSB of full-
scale error. If a larger filter resistor must be used, errors
can be eliminated by increasing the cycle time.
LTC1096/LTC1096L
LTC1098/LTC1098L
Input Leakage Current
Input leakage currents can also create errors if the source
resistance gets too large. For instance, the maximum
input leakage specification of 1 A (at 125 C) flowing
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 temperatures because leakage drops rapidly (see
typical curve of Input Channel Leakage Current vs Tem-
perature).
REFERENCE INPUTS
The voltage on the reference input of the LTC1096 defines
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 capaci-
20
A
PPLICATI
V
IN
IN
. When running at the minimum cycle time
R
DC
FILTER
Figure 9. RC Input Filtering
O
= 25pF(V
U
I
DC
S
C
FILTER
I FOR ATIO
U
IN
/t
CYC
“+”
“–”
LTC1098
) and is roughly pro-
LTC1096/8 • F9
W
F
(e.g., 1 F), the
IN
= 5V. In this
U
tive 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
2 s 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
reference 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 fac-
tors must be considered when operating at low V
values.
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 fixed 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 related to
Figure 10. Reference Input Equivalent Circuit
R
V
OUT
REF
REF
GND
5
4
+
CC
supply and reference are internally
REF
EVERY CLK CYCLE
R
ON
REF
5pF TO 30pF
LTC1096
because of the
LTC1096/8 • F10
REF