LTC1096 Linear Technology, LTC1096 Datasheet - Page 21

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

A
reference voltage for a typical value of V
a V
0.5LSB with a 1V reference and 2.5LSBs 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 LTC1096.
Noise with Reduced V
The total input referred noise of the LTC1096 can be
reduced to approximately 1mV peak-to-peak using a ground
plane, good bypassing, good layout techniques and mini-
mizing noise on the reference inputs. This noise is insig-
nificant 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 1mV noise is only
0.05LSB peak-to-peak. In this case, the LTC1096 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
1V reference, this same 1mV noise is 0.25LSB 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 200mV, the 1mV
noise becomes equal to 1.25LSBs and a stable code may
be difficult to achieve. 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
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 LTC1096 internal comparator over-
drive is reduced. Therefore, it may be necessary to
reduce the maximum CLK frequency when low values
of V
Input Divider
It is OK to use an input divider on the reference input of the
LTC1096 as long as the reference input can be made to
PPLICATI
OS
REF
of 2mV which is 0.1LSB with a 5V reference becomes
are used.
O
U
S
REF
I FOR ATIO
U
CC
REF
, V
W
REF
OS
or V
. For example,
IN
U
) will add
settle within the bit time at which the clock is running.
When using a larger value resistor divider on the reference
input the “–” input should be matched with an equivalent
resistance.
Bypassing Reference Input with Divider
Bypassing the reference input with a divider is also pos-
sible. However, care must be taken to make sure that the
DC voltage on the reference input will not drop too much
below the intended reference voltage.
AC PERFORMANCE
Two commonly used figures of merit for specifying the
dynamic performance of the ADCs in digital signal pro-
cessing applications are the signal-to-noise ratio (SNR)
and the effective number of bits (ENOBs).
Signal-to-Noise Ratio
The signal-to-noise ratio (SNR) is the ratio between the
RMS amplitude of the fundamental input frequency to
the RMS amplitude of all other frequency components at
the A/D output. This includes distortion as well as noise
products and for this reason it is sometimes referred to
as signal-to-noise + distortion [S/(N + D)]. The output is
band limited to frequencies from DC to one half the
sampling frequency. Figure 11 shows spectral content
from DC to 15.625kHz which is 1/2 the 31.25kHz sam-
pling rate.
Figure 11. This Clean FFT of an 11.8kHz Input Shows
Remarkable Performance for an ADC That Draws Only 100 A
When Sampling at the 31.25kHz Rate
–100
–110
–120
–20
–30
–40
–50
–70
–10
–60
–80
–90
0
0
f
f
SAMPLE
IN
= 11.8kHz
2
= 31.25kHz
4
LTC1096/LTC1096L
LTC1098/LTC1098L
FREQUENCY (kHz)
6
8
10
12
LTC1096/8 • F11
14
16
21