LTC1289 Linear Technology, LTC1289 Datasheet - Page 20

• Manufacturer Part Number: LTC1289
• Description: 3 Volt Single Chip 12-Bit Data Acquisition System
• Manufacturer: Linear Technology

LTC1289
2. It is recommended that REF
6. Reduced Reference Operation
The effective resolution of the LTC1289 can be increased by
reducing the input span of the converter. The LTC1289
exhibits good linearity and gain over a wide range of
reference voltages (see typical curves of Linearity and Gain
Error vs Reference Voltage). However, care must be taken
when operating at low values of V
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
20
A
PPLICATI
Figure 16. Poor Reference Settling Can Cause A/D Errors
the analog ground plane. If REF
other than ground, the voltage must not change during
a conversion cycle. This voltage must also be free of
noise and ripple with respect to analog ground.
Figure 15. Adequate Reference Settling
O
U
HORIZONTAL: 1 s/DIV
HORIZONTAL: 1 s/DIV
S
I FOR ATIO
U
REF
–
input be tied directly to
REF
–
because of the reduced
is biased at a voltage
W
values:
U
Offset with Reduced V
The offset of the LTC1289 has a larger effect on the output
code when the A/D 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
reference voltage for a typical value of V
V
becomes 0.4LSB with a 1.25V reference. If this offset is
unacceptable, it can be corrected digitally by the receiving
system or by offsetting the “–” input to the LTC1289.
Noise with Reduced V
The total input referred noise of the LTC1289 can be reduced
to approximately 200 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 2.5V reference but will become a larger fraction of an LSB
as the size of the LSB is reduced. The typical curve of Noise
Error vs Reference Voltage shows the LSB contribution of
this 200 V of noise.
For operation with a 2.5 reference, the 200 V noise is only
0.32LSB peak-to-peak. In this case, the LTC1289 noise will
contribute virtually no uncertainty to the output code. How-
ever, 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 1.25V reference, this
same 200 V noise is 0.64LSB peak-to-peak. This will re-
duce the range of input voltages over which a stable output
code can be achieved by 0.64LSB. In this case averaging
readings 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 0.1mV which is 0.2LSB with a 2.5V reference
REF
REF
CC
, V
REF
OS
. For example, a
, V
IN
or V
–
) will