LTC2487
applicaTions inForMaTion
+
–
amplifiers. The remaining common mode input current
is eliminated by either balancing the differential input im-
pedances or setting the common mode input equal to the
common mode reference (see the Automatic Differential
Input Current Cancellation section). This unique architec-
ture does not require on-chip buffers, thereby enabling
REF and REF pins covers the entire operating range of
the device (GND to V ). For correct converter operation,
CC
+
–
V
must be positive (REF > REF ).
REF
The LTC2487 differential reference input range is 0.1V to
+
V . For the simplest operation, REF can be shorted to
CC
CC
–
V
and REF can be shorted to GND. The converter out-
signals to swing beyond ground and V . Moreover, the
CC
put noise is determined by the thermal noise of the front
end circuits and, as such, its value in nanovolts is nearly
constant with reference voltage. A decrease in reference
voltagewillnotsignificantlyimprovetheconverter’seffec-
tive resolution. On the other hand, a decreased reference
will improve the converter’s overall INL performance.
cancellation does not interfere with the transparent offset
and full-scale auto-calibration and the absolute accuracy
(full scale + offset + linearity + drift) is maintained even
with external RC networks.
Power-Up Sequence
The LTC2487 automatically enters an internal reset state
Input Voltage Range
when the power supply voltage, V , drops below a
CC
The analog inputs are truly differential with an absolute,
commonmoderangefortheCH0-CH3andCOMinputpins
extending from GND – 0.3V to V + 0.3V. Outside these
threshold of approximately 2.0V. This feature guarantees
the integrity of the conversion result and input channel
selection.
CC
limits, the ESD protection devices begin to turn on and the
errorsduetoinputleakagecurrentincreaserapidly.Within
these limits, the LTC2487 converts the bipolar differential
WhenV risesabovethisthreshold,theconvertercreates
CC
an internal power-on-reset (POR) signal with a duration
of approximately 4ms. The POR signal clears all internal
registers. The conversion immediately following a POR
+
–
+
–
input signal V = IN – IN (where IN and IN are the
IN
selected input channels), from –FS = –0.5 • V /Gain
REF
+
+
–
cycle is performed on the input channels IN = CH0 and
to +FS = 0.5 • V /Gain where V
= REF – REF .
REF
REF
–
IN = CH1 with simultaneous 50Hz/60Hz rejection, 1x
Outside this range, the converter indicates the overrange
or the underrange condition using distinct output codes
(see Table 1).
output rate, and gain = 1. The first conversion following a
PORcycleisaccuratewithinthespecificationofthedevice
if the power supply voltage is restored to (2.7V to 5.5V)
before the end of the POR interval. A new input channel,
rejection mode, speed mode, temperature selection or
gain can be programmed into the device during this first
data input/output cycle.
In order to limit any fault current, resistors of up to 5k
may be added in series with the input. The effect of series
resistanceontheconverteraccuracycanbeevaluatedfrom
the curves presented in the Input Current/Reference Cur-
rent sections. In addition, series resistors will introduce a
temperaturedependenterrorduetoinputleakagecurrent.
A 1nA input leakage current will develop a 1ppm offset
Reference Voltage Range
Thisconverteracceptsatrulydifferentialexternalreference
voltage.Theabsolute/commonmodevoltagerangeforthe
error on a 5k resistor if V
strong temperature dependency.
= 5V. This error has a very
REF
2487fa
ꢀꢂ
Linear [ Linear ]
