LTC2488 LINER [Linear Technology], LTC2488 Datasheet - Page 23

• Manufacturer Part Number: LTC2488
• Description: 16-Bit 2-/4-Channel ?? ADC with Easy Drive Input Current Cancellation
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Figure 15. INL vs Differential Input Voltage and Reference
Source Resistance for C
Figure 13. +FS Error vs R
Figure 14. –FS Error vs R
–400
–100
–200
–300
–500
500
400
200
100
300
–10
10
–2
–4
–6
–8
0
0
8
6
4
2
0
– 0.5
0
0
V
V
V
V
F
T
V
V
V
V
F
T
V
V
V
T
C
O
O
A
CC
REF
IN
IN
A
CC
REF
IN
IN
CC
REF
IN(CM)
A
REF
= GND
= 25°C
C
= GND
= 25°C
+
–
+
–
= 25°C
= 5V
REF
= 5V
= 5V
= 1.25V
= 3.75V
= 3.75V
= 1.25V
= 5V
= 5V
= 5V
= 10µF
– 0.3
200
200
= 1µF, 10µF
= 2.5V
REF
– 0.1
400
R
400
R
SOURCE
SOURCE
V
SOURCE
SOURCE
> 1µF
IN
/V
C
REF
REF
600
(Ω)
600
(Ω)
0.1
= 1µF, 10µF
at V
at V
C
C
REF
R = 500Ω
R = 100Ω
REF
C
C
REF
REF
R = 1k
REF
REF
800
800
= 0.01µF
0.3
= 0.01µF
= 0.1µF
= 0.1µF
(Large C
(Large C
2488 F13
2488 F14
2488 F15
1000
1000
0.5
REF
REF
)
)
Normal Mode Rejection and Anti-aliasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital fi ltering. Combined
with a large oversample ratio, the LTC2488 signifi cantly
simplifi es anti-aliasing fi lter requirements. Additionally,
the input current cancellation feature allows external low
pass fi ltering without degrading the DC performance of
the device.
The SINC
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (f
sampling frequency is f
its internal oscillator and f
with an external oscillator of frequency F
When using the internal oscillator, the LTC2488 is designed
to reject line frequencies. As shown in Figure 16, rejection
nulls occur at multiples of frequency f
for simultaneous 50Hz/60Hz rejection. Multiples of the
modulator sampling rate (f
to 15dB (see Figure 17), if noise sources are present at
these frequencies anti-aliasing will reduce their effects.
The user can expect to achieve this level of performance
using the internal oscillator, as shown in Figure 18.
Measured values of normal mode rejection are shown
superimposed over the theoretical values.
Traditional high order delta-sigma modulators suffer
from potential instabilities at large input signal levels. The
Figure 16. Input Normal Mode Rejection at DC
4
–100
–110
–120
digital fi lter provides excellent normal mode
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
f
N
INPUT SIGNAL FREQUENCY (Hz)
2f
N
S
= 15,360Hz while operating with
3f
N
S
S
= f
= F
4f
N
N
EOSC
5f
• 256) only reject noise
N
f
N
= f
6f
/20 when operating
N
N
EOSC/5120
S
, where f
LTC2488
). The modulator
7f
EOSC
N
2488 F16
8f
N
.
N
23
= 55Hz
2488f