LTC2480 Datasheet, PDF (27/40 Pages) Linear Technology – 16-Bit ADC with Easy Drive

LTC2480
APPLICATIO S I FOR ATIO
occurs. In this case, no errors are introduced and direct
digitization of the sensor is possible.
For many applications, the sensor output impedance com-
bined with external bypass capacitors produces RC time
constants much greater than the 580ns required for 1ppm
accuracy. For example, a 10kΩ bridge driving a 0.1µF
bypass capacitor has a time constant an order of magni-
tude greater than the required maximum. Historically,
settling issues were solved using buffers. These buffers
led to increased noise, reduced DC performance (Offset/
Drift), limited input/output swing (cannot digitize signals
near ground or V_CC), added system cost and increased
power. The LTC2480 uses a proprietary switching algo-
rithm that forces the average differential input current to
zero independent of external settling errors. This allows
accurate direct digitization of high impedance sensors
without the need of buffers. Additional errors resulting
from mismatched leakage currents must also be taken into
account.
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_INCM) and the common mode reference
voltage (V_REFCM).
In applications where the input common mode voltage is
equal to the reference common mode voltage, as in the
case of a balance bridge type application, both the differ-
ential and common mode input current are zero. The
accuracy of the converter is unaffected by settling errors.
Mismatches in source impedances between IN⁺ and IN^–
also do not affect the accuracy.
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_INCM and V_REFCM. For a reference
common mode of 2.5V and an input common mode of
1.5V, the common mode input current is approximately
0.74µA (in simultaneous 50Hz/60Hz rejection mode). This
common mode input current has no effect on the accuracy
if the external source impedances tied to IN⁺ and IN^– are
matched. Mismatches in these source impedances lead to
a fixed offset error but do not affect the linearity or full-
scale reading. A 1% mismatch in 1kΩ source resistances
leads to a 15ppm shift (74µV) in offset voltage.
I_REF+
V_REF⁺
I_IN+
V_IN⁺
I_IN–
V_IN–
I_REF–
GND
V_CC
I_LEAK
R_SW (TYP)
10k
I_LEAK
V_CC
I_LEAK
R_SW (TYP)
10k
I_LEAK
V_CC
I_LEAK
R_SW (TYP)
10k
I_LEAK
V_CC
I_LEAK
R_SW (TYP)
10k
I_LEAK
2480 F11
C_EQ
12pF
(TYP)
( ) ( ) ^{I IN+}
= I IN^–
= ^{VIN(CM) − VREF(CM)}
AVG
AVG
0.5 •R_EQ
( ) ^{( ) I REF+}
= ^{1.5 • VREF − VINCM + VREFCM} −
V_IN 2
− ^{0.5 • VREF • DT}
1.5V_REF +
≅
V_REF(CM) – V_IN(CM)
–
V_IN2
AVG
0.5 •R_EQ
V_REF • R_EQ
R_EQ
0.5 • R_EQ
V_REF • R_EQ
where:
V_REFCM
=
⎛ _VREF+⎞
⎜
⎝
2
⎟
⎠
V_IN = IN⁺ − IN^−
V_INCM
=
⎛
⎝⎜
IN⁺
+ IN^−
2
⎞
⎠⎟
R_EQ = 2.71MΩ INTERNAL OSCILLATOR 60Hz MODE
R_EQ = 2.98MΩ INTERNAL OSCILLATOR 50Hz AND 60Hz MODE
( ) ^REQ ^{= 0.833 •1012 / f} EOSC ^{EXTERNAL OSCILLATOR}
D_T IS THE DENSITY OF A DIGITAL TRANSITION AT THE MODULATOR OUTPUT
WHERE REF^– IS INTERNALLY TIED TO GND
SWITCHING FREQUENCY
f_SW = 123kHz INTERNAL OSCILLATOR
f_SW = 0.4 • f_EOSC EXTERNAL OSCILLATOR
Figure 11. LTC2480 Equivalent Analog Input Circuit
2480f
27