LTC2412 Linear Technology, LTC2412 Datasheet - Page 30

• Manufacturer Part Number: LTC2412
• Description: 2-Channel Differential Input 24-Bit No Latency DS ADC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC2412
30
Figure 29. Resolution (Noise
vs Output Data Rate and Reference Voltage
Figure 30. Resolution (INL
vs Output Data Rate and Reference Voltage
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
–4.5
–5.0
–5.5
–6.0
0.0
22
20
18
16
14
12
10
24
23
22
21
20
19
18
17
16
15
14
13
12
8
Figure 31. Input Signal Bandwidth
Using the Internal Oscillator
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
T
V
REF
V
–0.5V • V
F
V
REF
V
V
F
T
RESOLUTION = LOG
O
O
A
CC
INCM
CC
INCM
IN
A
10 20 30 40 50 60 70 80 90 100
10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
= 25 C
= EXTERNAL OSCILLATOR
= EXTERNAL OSCILLATOR
= 25 C
OUTPUT DATA RATE (READINGS/SEC)
= 0V
–
–
= 5V
= 5V
= GND
= GND
V
= 0.5 • REF
= 2.5V
REF
U
REF
F
O
= 2.5V
= HIGH
< V
V
REF
IN
+
U
< 0.5 • V
= 2.5V
2
(V
V
V
REF
RESOLUTION =
LOG
REF
REF
/NOISE
REF
F
= 5V
2
MAX
O
= 5V
(V
= LOW
RMS
REF
W
RMS
/INL
1LSB)
)
MAX
1LSB)
2412 F30
2412 F31
2412 F29
)
U
Input Bandwidth
The combined effect of the internal Sinc
of the analog and digital autocalibration circuits deter-
mines the LTC2412 input bandwidth. When the internal
oscillator is used with the notch set at 60Hz (F
the 3dB input bandwidth is 3.63Hz. When the internal
oscillator is used with the notch set at 50Hz (F
the 3dB input bandwidth is 3.02Hz. If an external conver-
sion clock generator of frequency f
F
Due to the complex filtering and calibration algorithms
utilized, the converter input bandwidth is not modeled very
accurately by a first order filter with the pole located at the
3dB frequency. When the internal oscillator is used, the
shape of the LTC2412 input bandwidth is shown in Fig-
ure 31 for F
oscillator of frequency f
LTC2412 input bandwidth can be derived from Figure 31,
F
f
The conversion noise (800nV
can be modeled by a white noise source connected to a
noise free converter. The noise spectral density is
62.75nV/ Hz for an infinite bandwidth source and
86.1nV/ Hz for a single 0.5MHz pole source. From these
numbers, it is clear that particular attention must be given
to the design of external amplification circuits. Such
circuits face the simultaneous requirements of very low
bandwidth (just a few Hz) in order to reduce the output
referred noise and relatively high bandwidth (at least
500kHz) necessary to drive the input switched-capacitor
network. A possible solution is a high gain, low bandwidth
amplifier stage followed by a high bandwidth unity-gain
buffer.
When external amplifiers are driving the LTC2412, the
ADC input referred system noise calculation can be simpli-
fied by Figure 32. The noise of an amplifier driving the
LTC2412 input pin can be modeled as a band limited white
noise source. Its bandwidth can be approximated by the
bandwidth of a single pole lowpass filter with a corner
frequency f
From Figure 32, using f
on the y-axis the noise equivalent bandwidth freq
EOSC
O
O
pin, the 3dB input bandwidth is 0.236 • 10
= LOW curve in which the horizontal axis is scaled by
/153600.
i
O
. The amplifier noise spectral density is n
= LOW and F
i
as the x-axis selector, we can find
EOSC
O
= HIGH. When an external
RMS
is used, the shape of the
EOSC
typical for V
is connected to the
4
digital filter and
–6
O
O
REF
= HIGH),
= LOW),
• f
i
EOSC
of the
= 5V)
2412f
.
i
.