LTC1594 Linear Technology, LTC1594 Datasheet - Page 17

• Manufacturer Part Number: LTC1594
• Description: 4- and 8-Channel/ Micropower Sampling 12-Bit Serial I/O A/D Converters
• Manufacturer: Linear Technology

Reduced Reference Operation
The effective resolution of the LTC1594/LTC1598 can be
increased by reducing the input span of the converters.
The LTC1594/LTC1598 exhibit good linearity and gain
over a wide range of reference voltages (see typical
curves Change in Linearity vs Reference Voltage and
Change in Gain vs Reference Voltage). However, care
must be taken when operating at low values of V
because of the reduced LSB step size and the resulting
higher accuracy requirement placed on the converters.
The following factors must be considered when operat-
ing at low V
1. Offset
2. Noise
3. Conversion speed (CLK frequency)
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 200nA (at 85 C) flowing
through a source resistance of 1.2k will cause a voltage
drop of 240 V or 0.2LSB. This error will be much
reduced at lower temperatures because leakage drops
rapidly (see typical curve Input Channel Leakage Current
vs Temperature).
REFERENCE INPUTS
The reference input of the LTC1594/LTC1598 is effec-
tively a 50k resistor from the time CS goes low to the end
of the conversion. The reference input becomes a high
impedance node at any other time (see Figure 9). Since
the voltage on the reference input defines the voltage
span of the A/D converter, the reference input should be
driven by a reference with low R
and LT1021) or a voltage source with low R
APPLICATIONS
Figure 9. Reference Input Equivalent Circuit
R
V
OUT
REF
REF
values:
REF
GND
1
4
U
+
INFORMATION
U
OUT
W
(ex. LT1004, LT1019
LTC1594
LTC1598
1594/98 F09
OUT
U
.
REF
Offset with Reduced V
The offset of the LTC1594/LTC1598 has a larger effect on
the output code when the ADCs are 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 Change in
Offset vs Reference Voltage shows how offset in LSBs is
related to reference voltage for a typical value of V
example, a V
reference becomes 0.5LSB with a 1V reference and
2.5LSBs with a 0.2V reference. If this offset is unaccept-
able, it can be corrected digitally by the receiving system
or by offsetting the “COM” input of the LTC1594/LTC1598.
Noise with Reduced V
The total input referred noise of the LTC1594/LTC1598
can be reduced to approximately 400 V peak-to-peak
using a ground plane, good bypassing, good layout
techniques and minimizing noise on the reference inputs.
This noise is insignificant with a 5V reference but will
become a larger fraction of an LSB as the size of the LSB
is reduced.
For operation with a 5V reference, the 400 V noise is only
0.33LSB peak-to-peak. In this case, the LTC1594/LTC1598
noise will contribute virtually no uncertainty to the output
code. However, for reduced references the noise may
become a significant fraction of an LSB and cause
undesirable jitter in the output code. For example, with a
2.5V reference this same 400 V noise is 0.66LSB peak-
to-peak. This will reduce the range of input voltages over
which a stable output code can be achieved by 1LSB. If
the reference is further reduced to 1V, the 400 V noise
becomes equal to 1.65LSBs and a stable code may be
difficult to achieve. In this case averaging multiple read-
ings may be necessary.
This noise data was taken in a very clean setup. Any setup
induced noise (noise or ripple on V
add to the internal noise. The lower the reference voltage
to be used the more critical it becomes to have a clean,
noise free setup.
OS
of 122 V which is 0.1LSB with a 5V
REF
REF
LTC1594/LTC1598
CC
, V
REF
or V
17
IN
OS
) will
. For