LTC2488_15 Datasheet, PDF (22/30 Pages) Linear Technology – 16-Bit 2-/4-Channel ADC with Easy Drive Input Current Cancellation

LTC2488
APPLICATIONS INFORMATION
INPUT
MULTIPLEXER
I_IN+
100Ω
IN+
I_IN–
100Ω
IN^–
I_REF+
REF⁺
I_REF–
REF^–
INTERNAL
SWITCH
NETWORK
10k
10k
10k
10k
C_EQ
12pF
( ) ( ) ^{I IN+}
= I IN^–
= ^{VIN(CM) − VREF(CM)}
AVG
AVG
0.5•R_EQ
( ) _{I REF+}
1.5V_REF + V_REF(CM) – V_IN(CM)
≈
–
V_I2N
( )_AVG
0.5 •R_EQ
V_REF •R_EQ
where :
V_REF =REF⁺ −REF^−
V_REF(CM)
=



REF⁺

– REF^−
2




V_IN =IN⁺ −IN^−,WHERE IN⁺ AND IN^− ARE THE SELECTED INPUT CHANNELS
V_IN(CM)

=



IN⁺
– IN^−
2




R_EQ = 2.98MΩ INTERNAL OSCILLATOR
( ) ^REQ ^{= 0.833•1012 /f}EOSC ^{EXTERNAL OSCILLATOR}
2488 F10
SWITCHING FREQUENCY
f_SW = 123kHz INTERNAL OSCILLATOR
f_SW = 0.4 • f_EOSC EXTERNAL OSCILLATOR
Figure 10. LTC2488 Equivalent Analog Input Circuit
larger source resistances. Source resistances up to 10k
may interface directly to the LTC2488 and settle completely;
however, the addition of external capacitors at the input
terminals in order to filter unwanted noise (anti-aliasing)
results in incomplete settling.
Automatic Differential Input Current Cancellation
In applications where the sensor output impedance is
low (up to 10kΩ with no external bypass capacitor or up
to 500Ω with 0.001µF bypass), complete settling of the
input occurs. In this case, no errors are introduced and
direct digitization is possible.
For many applications, the sensor output impedance
combined with external input bypass capacitors produces
RC time constants much greater than the 580ns required
for 1ppm accuracy. For example, a 10kΩ bridge driving a
0.1µF capacitor has a time constant an order of magnitude
greater than the required maximum.
The LTC2488 uses a proprietary switching algorithm that
forces the average differential input current to zero indepen-
dent of external settling errors. This allows direct digitization
of high impedance sensors without the need of buffers.
The switching algorithm forces the average input current
on the positive input (I_IN+) to be equal to the average input
current on the negative input (I_IN–). Over the complete
conversion cycle, the average differential input current
(I_IN+ – I_IN–) is zero. While the differential input current is
zero, the common mode input current (I_IN+ + I_IN–)/2 is
proportional to the difference between the common mode
input voltage (V_IN(CM)) and the common mode reference
voltage (V_REF(CM)).
In applications where the input common mode voltage is
equal to the reference common mode voltage, as in the
case of a balanced bridge, both the differential and com-
mon mode input currents are zero. The accuracy of the
converter is not compromised by settling errors.
In applications where the input common mode voltage is
constant but different from the reference common mode
voltage, the differential input current remains zero while
the common mode input current is proportional to the
difference between V_IN(CM) and V_REF(CM). For a reference
common mode voltage of 2.5V and an input common
mode of 1.5V, the common mode input current is ap-
proximately 0.74µA. This common mode input current
does not degrade the accuracy if the source impedances
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