LTC1288CS8 Linear Technology, LTC1288CS8 Datasheet - Page 18

• Manufacturer Part Number: LTC1288CS8
• Description: IC A/D CONV SAMPLING 12BIT 8SOIC
• Manufacturer: Linear Technology

Specifications of LTC1288CS8

• Number Of Bits: 12
• Sampling Rate (per Second): 6.6k
• Data Interface: MICROWIRE™, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 630µW
• Voltage Supply Source: Single Supply
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

LTC1285/LTC1288
APPLICATION INFORMATION
Voltage). However, care must be taken when operating at
low values of V
and the resulting higher accuracy requirement placed on
the converter. The following factors must be considered
when operating at low V
Offset with Reduced V
The offset of the LTC1285 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 Change in Offset vs
Reference Voltage shows how offset in LSBs is related to
reference voltage for a typical value of V
a V
becomes 1LSB with a 1V reference and 5LSBs with a 0.2V
reference. If this offset is unacceptable, it can be corrected
digitally by the receiving system or by offsetting the “–”
input of the LTC1285.
Noise with Reduced V
The total input referred noise of the LTC1285 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 2.5V reference but will become a
larger fraction of an LSB as the size of the LSB is reduced.
For operation with a 2.5V reference, the 400 V noise is
only 0.66LSB peak-to-peak. In this case, the LTC1285
noise will contribute a little bit of 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 1.25V reference this same 400 V noise is 1.32LSB
peak-to-peak. This will reduce the range of input volt-
ages over which a stable output code can be achieved by
1LSB. If the reference is further reduced to 1V, the 400 V
18
1. Offset
2. Noise
3. Conversion speed (CLK frequency)
OS
of 122 V which is 0.2LSB with a 2.5V reference
REF
because of the reduced LSB step size
U
REF
REF
REF
U
values:
W
OS
. For example,
U
noise becomes equal to 3.3LSBs and a stable code may
be difficult to achieve. In this case averaging multiple
readings may be necessary.
This noise data was taken in a very clean setup. Any setup
induced noise (noise or ripple on V
to the internal noise. The lower the reference voltage to be
used the more critical it becomes to have a clean, noise free
setup.
Conversion Speed with Reduced V
With reduced reference voltages, the LSB step size is
reduced and the LTC1285 internal comparator over-
drive is reduced. Therefore, it may be necessary to
reduce the maximum CLK frequency when low values
of V
DYNAMIC PERFORMANCE
The LTC1285/LTC1288 have exceptional sampling capa-
bility. Fast Fourier Transform (FFT) test techniques are
used to characterize the ADC’s frequency response, dis-
tortion and noise at the rated throughput. By applying a
low distortion sine wave and analyzing the digital output
using an FFT algorithm, the ADC’s spectral content can be
examined for frequencies outside the fundamental. Figure
11 shows a typical LTC1285 plot.
Figure 11. LTC1285 Non-Averaged, 4096 Point FFT Plot
REF
are used.
– 100
– 120
– 60
– 20
– 40
– 80
0
0
T
V
V
f
f
f
IN
CLK
SMPL
A
CC
REF
= 25°C
= 3.05kHz
0.5
= 2.7V
= 120kHz
= 2.5V
= 7.5kHz
1.0
FREQUENCY (kHz)
1.5
2.0
2.5
CC
3.0
LTC1285/88 • TPC16
REF
, V
3.5
REF
or V
4.0
IN
) will add