For a simple approximation, the source impedance RS
參數(shù)資料
型號(hào)： LTC2485CDD#TRPBF
廠商： Linear Technology
文件頁(yè)數(shù)： 15/40頁(yè)
文件大?。?/td> 0K
描述： IC ADC 24BIT I2C 10-DFN
標(biāo)準(zhǔn)包裝： 2,500
位數(shù)： 24
采樣率（每秒）： 6.8
數(shù)據(jù)接口： I²C，串行
轉(zhuǎn)換器數(shù)目： 1
功率耗散（最大）： 480µW
電壓電源： 單電源
工作溫度： 0°C ~ 70°C
安裝類(lèi)型： 表面貼裝
封裝/外殼： 10-WFDFN 裸露焊盤(pán)
供應(yīng)商設(shè)備封裝： 10-DFN（3x3）
包裝： 帶卷 (TR)
輸入數(shù)目和類(lèi)型： 1 個(gè)差分，雙極
配用： DC956A-ND - BOARD DELTA SIGMA ADC LTC2485
LTC2485
22
2485fc
APPLICATIONS INFORMATION
For a simple approximation, the source impedance RS
driving an analog input pin (IN+, IN–, REF+ or REF–) can be
considered to form, together with RSW and CEQ (see Fig-
ure 12), a rst order passive network with a time constant
τ = (RS + RSW) CEQ. The converter is able to sample the
input signal with better than 1ppm accuracy if the sampling
period is at least 14 times greater than the input circuit time
constant τ. The sampling process on the four input analog
pins is quasi-independent so each time constant should be
considered by itself and, under worst-case circumstances,
the errors may add.
When using the internal oscillator, the LTC2485’s front-end
switched-capacitor network is clocked at 123kHz corre-
sponding to an 8.1μs sampling period. Thus, for settling
errors of less than 1ppm, the driving source impedance
should be chosen such that τ ≤ 8.1μs/14 = 580ns. When an
external oscillator of frequency fEOSC is used, the sampling
period is 2.5/fEOSC and, for a settling error of less than
1ppm, τ ≤ 0.178/fEOSC.
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 of the sensor is possible.
For many applications, the sensor output impedance
combined 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 magnitude
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 VCC), added system cost and increased power.
The LTC2485 uses a proprietary switching algorithm that
forces the average differential input current to zero inde-
pendent of external settling errors. This allows accurate
direct digitization of high impedance sensors without the
need of buffers (see Figures 13 to 15). 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 (IIN+) to be equal to the average input
current on the negative input (IIN–). Over the complete
conversion cycle, the average differential input current
(IIN+ – IIN–) is zero. While the differential input current
is zero, the common mode input current (IIN++ IIN–)/2 is
proportional to the difference between the common mode
input voltage (VINCM) and the common mode reference
voltage (VREFCM).
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
differential 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 VINCM and VREFCM. 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 xed 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.
In applications where the common mode input voltage
varies as a function of input signal level (single-ended
input, RTDs, half bridges, current sensors, etc.), the com-
mon mode input current varies proportionally with input
voltage. For the case of balanced input impedances, the
common mode input current effects are rejected by the
large CMRR of the LTC2485 leading to little degradation in
accuracy. Mismatches in source impedances lead to gain
errors proportional to the difference between the common
mode input voltage and the common mode reference
voltage. 1% mismatches in 1kΩ source resistances lead
to worst-case gain errors on the order of 15ppm or 1LSB
(for 1V differences in reference and input common mode
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