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LTC2489CDE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LTC2489
19
2489fb
For more information www.linear.com/LTC2489
full-scale error of approximately 0.5ppm (while operating
with the internal oscillator) (see Figures 12 and 13). If
the input common mode voltage is equal to the reference
common mode voltage, a linearity error of approximately
0.67ppm per 100W
of reference resistance results (see
Figure 14). In applications where the input and reference
common mode voltages are different, the errors increase.
A 1V difference in between common mode input and
common mode reference results in a 6.7ppm INL error
for every 100W of reference resistance.
In addition to the reference sampling charge, the reference
ESD protection diodes have a temperature dependent leak
-
age current. This leakage current, nominally 1nA (±10nA
max) results in a small, gain error
. A 100
W reference
resistance will create a 0.5µV full-scale error.
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital filtering. Combined
with a large oversample ratio, the LTC2489 significantly
simplifies antialiasing filter requirements. Additionally,
the input current cancellation feature allows external low
pass filtering without degrading the DC performance of
the device.
The SINC
4
digital filter provides excellent normal mode
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (f
S
). The modulator
sampling frequency is f
S
= 15,360Hz while operating with
its internal oscillator and f
S
= f
EOSC
/20 when operating
with an external oscillator of frequency f
EOSC
.
applicaTions inForMaTion
Figure 14. INL vs Differential Input Voltage and
Reference Source Resistance for C
REF
> 1µF
Figure 12. +FS Error vs R
SOURCE
at V
REF
(Large C
REF
) Figure 13. –FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2489 F12
200
100
0
200
400
600
1000
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 3.75V
V
IN
–
= 1.25V
f
O
= GND
T
A
= 25°C
C
REF
= 1µF, 10µF
C
REF
= 0.1µF
C
REF
= 0.01µF
R
SOURCE
(Ω)
0
–200
–100
0
800
2489 F13
–300
–400
–500
200
400
600
1000
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 1.25V
V
IN
–
= 3.75V
f
O
= GND
T
A
= 25°C
C
REF
= 1µF, 10µF
C
REF
= 0.1µF
C
REF
= 0.01µF
V
IN
/V
REF
–0.5
INL (ppm OF V
REF
)
2
6
10
0.3
2489 F14
–2
–6
0
4
8
–4
–8
–10
–0.3
–0.1
0.1
0.5
V
CC
= 5V
V
REF
= 5V
V
IN(CM)
= 2.5V
T
A
= 25°C
C
REF
= 10µF
R = 1k
R = 100Ω
R = 500Ω
LTC2489
20
2489fb
For more information www.linear.com/LTC2489
When using the internal oscillator, the LTC2489 is designed
to reject line frequencies. As shown in Figure 15, rejection
nulls occur at multiples of frequency f
N
, where f
N
= 55Hz
for simultaneous 50Hz/60Hz rejection. Multiples of the
modulator sampling rate (f
S
= f
N
• 256) only reject noise
to 15dB (see Figure 16); if noise sources are present at
these frequencies antialiasing will reduce their effects.
The user can expect to achieve this level of performance us-
ing the internal oscillator, as shown in Figure 17. 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
proprietar
y architecture used for the LTC2489 third order
modulator resolves this problem and guarantees stability
with input signals 150% of full scale. In many industrial
applications, it is not uncommon to have microvolt level
signals superimposed over unwanted error sources with
several volts if peak-to-peak noise. Figure 18 shows mea-
surement results for the rejection of a 7.5V peak-to-peak
noise source (150% of full scale) applied to the L
TC2489.
This curve shows that the rejection performance is main-
tained even in extremely noisy environments.
Output Data Rate
When using its internal oscillator, the L
TC2489 produces
up to 7.5 samples per second (sps) with a notch frequency
of 60Hz. The actual output data rate depends upon the length
of the sleep and data output cycles which are controlled
by the user and can be made insignificantly short. When
operating with an external conversion clock (f
O
connected
applicaTions inForMaTion
Figure 16. Input Normal Mode Rejection at f
S
= 256 • f
N
Figure 15. Input Normal Mode Rejection at DC
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2489 F15
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
0 2f
N
3f
N
4f
N
5f
N
6f
N
7f
N
8f
N
f
N
= f
EOSC/5120
INPUT SIGNAL FREQUENCY (Hz)
250f
N
252f
N
254f
N
256f
N
258f
N
260f
N
262f
N
INPUT NORMAL MODE REJECTION (dB)
2489 F16
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
= f
EOSC/5120
INPUT FREQUENCY (Hz)
0
12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200
NORMAL MODE REJECTION (dB)
2489 F17
0
–20
–40
–60
–80
–100
–120
V
CC
= 5V
V
REF
= 5V
V
IN(CM)
= 2.5V
V
IN(P-P)
= 5V
T
A
= 25°C
MEASURED DATA
CALCULATED DATA
INPUT FREQUENCY (Hz)
0
15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240
NORMAL MODE REJECTION (dB)
2489 F18
0
–20
–40
–60
–80
–100
–120
V
CC
= 5V
V
REF
= 5V
V
IN(CM)
= 2.5V
T
A
= 25°C
V
IN(P-P)
= 5V
V
IN(P-P)
= 7.5V
(150% OF FULL SCALE)
Figure 17. Input Normal Mode Rejection vs Input Frequency with
Input Perturbation of 100% (50Hz/60Hz Notch)
Figure 18. Measure Input Normal Mode Rejection vs Input
Frequency with Input Perturbation of 150% (60Hz Notch)
LTC2489
21
2489fb
For more information www.linear.com/LTC2489
to an external oscillator), the LTC2489 output data rate
can be increased. The duration of the conversion cycle is
41036/f
EOSC
. If f
EOSC
= 307.2kHz, the converter behaves
as if the internal oscillator is used.
An increase in f
EOSC
over the nominal 307.2kHz will trans-
late into a proportional increase in the maximum output
data rate (up to a maximum of 100sps). The increase in
output rate leads to degradation in offset, full-scale error,
and effective resolution as well as a shift in frequency
rejection.
A change in f
EOSC
results in a proportional change in the
internal notch position. This leads to reduced differential
mode rejection of line frequencies. The common mode
rejection of line frequencies remains unchanged, thus fully
differential input signals with a high degree of symmetry
on both the IN
+
and IN
–
pins will continue to reject line
frequency noise.
An increase in f
EOSC
also increases the effective dynamic
input and reference current. External RC networks will
continue to have zero differential input current, but the
time required for complete settling (580ns for f
EOSC
=
307.2kHz) is reduced, proportionally.
Once the external oscillator frequency is increased above
1MHz (a more than 3X increase in output rate) the ef
-
fectiveness of internal auto calibration circuits begins to
degrade. This results in larger offset errors, full-scale errors,
and decreased resolution, as shown in Figures 19 to 26.
Easy Drive ADCs Simplif
y Measurement of High
Impedance Sensors
Delta-Sigma ADCs, with their high accuracy and high noise
immunity, are ideal for directly measuring many types
of sensors. Nevertheless, input sampling currents can
overwhelm high source impedances or low-bandwidth,
micropower signal conditioning circuits. The LTC2489
applicaTions inForMaTion
solves this problem by balancing the input currents, thus
simplifying or eliminating the need for signal conditioning
circuits.
A common application for a delta-sigma ADC is thermistor
measurement. Figure 28 shows two examples of thermis
-
tor digitization benefiting from the Easy Drive technology.
The first cir
cuit (applied to input channels CH0 and CH1)
uses balanced reference resistors in order to balance the
common mode input/reference voltage and balance the
differential input sour
ce resistance. If reference resistors
R1 and R4 are exactly equal, the input current is zero and
no errors result. If these resistors have a 1% tolerance,
the maximum error in measured resistance is 1.6W due
to a shift in common mode voltage; far less than the 1%
error of the reference resistors themselves. No amplifier
is required, making this an ideal solution in micropower
applications.
Easy Drive also enables very low power, low bandwidth
amplifiers to drive the input to the LTC2489. As shown in
Figure 28, CH2 is driven by the LT1494. The LT1494 has
excellent DC specs for an amplifier with 1.5µA supply
current (the maximum offset voltage is 150µV and the
open loop gain is 100,000). Its 2kHz bandwidth makes
it unsuitable for driving conventional delta sigma ADCs.
Adding a 1kW, 0.1µF filter solves this problem by providing
a charge reservoir that supplies the LTC2489 instantaneous
current, while the 1k resistor isolates the capacitive load
from the LT1494.
Conventional delta sigma ADCs input sampling current
lead to DC errors as a result of incomplete settling in the
external RC network.
The Easy Drive technology cancels the differential input
current. By balancing the negative input (CH3) with a 1kW,
0.1µF network errors due to the common mode input cur
-
rent are cancelled.
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LTC2489CDE#PBF

Mfr. #:
Buy LTC2489CDE#PBF
Manufacturer:
Analog Devices Inc.
Description:
Analog to Digital Converters - ADC 16-bit, 4-ch I2C Delta Sigma ADC
Lifecycle:
New from this manufacturer.
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