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参数资料
| 型号： | DC951A |
| 厂商： | Linear Technology |
| 文件页数： | 17/40页 |
| 文件大小： | 0K |
| 描述： | BOARD DELTA SIGMA ADC LTC2481 |
| 软件下载： | QuikEval System |
| 设计资源： | DC951A Design File DC951A Schematic |
| 标准包装： | 1 |
| 系列： | Easy Drive™, QuikEval™ |
| ADC 的数量： | 1 |
| 位数： | 16 |
| 采样率（每秒）： | 7.5 |
| 数据接口： | I²C，串行，SPI? |
| 工作温度： | 0°C ~ 70°C |
| 已用 IC / 零件： | LTC2481 |
| 已供物品： | 板 |
| 相关产品： | LTC2481HDD#TRPBF-ND - IC ADC DELTA SIGMA 16BIT 10DFN LTC2481HDD#PBF-ND - IC ADC 16BIT I2C 7.5SPS 10DFN LTC2481CDD#TRPBF-ND - IC ADC 16BIT I2C 10-DFN LTC2481IDD#TRPBF-ND - IC ADC 16BIT I2C 10-DFN LTC2481IDD#PBF-ND - IC ADC 16BIT I2C 10-DFN LTC2481CDD#PBF-ND - IC ADC 16BIT I2C 10-DFN |
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LTC2481
24
2481fc
APPLICATIONS INFORMATION
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 LTC2481 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
voltage). Table 7 summarizes the effects of mismatched
source impedance and differences in reference/input
common mode voltages.
Table 7. Suggested Input Conguration for LTC2481
BALANCED INPUT
RESISTANCES
UNBALANCED INPUT
RESISTANCES
Constant
VIN(CM) – VREF(CM)
CEXT > 1nF at Both IN+
and IN–. Can Take Large
Source Resistance with
Negligible Error
CEXT > 1nF at Both IN+
and IN–. Can Take Large
Source Resistance.
Unbalanced Resistance
Results in an Offset Which
Can be Calibrated
Varying
VIN(CM) – VREF(CM)
CEXT > 1nF at Both IN+
and IN–. Can Take Large
Source Resistance with
Negligible Error
Minimize IN+ and
IN– Capacitors and Avoid
Large Source Impedance
(< 5k Recommended)
The magnitude of the dynamic input current depends upon
the size of the very stable internal sampling capacitors and
upon the accuracy of the converter sampling clock. The
accuracy of the internal clock over the entire temperature
and power supply range is typically better than 0.5%. Such
a specication can also be easily achieved by an external
clock. When relatively stable resistors (50ppm/°C) are
used for the external source impedance seen by IN+ and
IN–, the expected drift of the dynamic current and offset
will be insignicant (about 1% of their respective values
over the entire temperature and voltage range). Even for
the most stringent applications, a one-time calibration
operation may be sufcient.
In addition to the input sampling charge, the input ESD
protection diodes have a temperature dependent leakage
current. This current, nominally 1nA (±10nA max), results
in a small offset shift. A 1k source resistance will create a
1μV typical and 10μV maximum offset voltage.
Reference Current
In a similar fashion, the LTC2481 samples the differential
reference pins REF+ and REF– transferring small amount
of charge to and from the external driving circuits thus
producing a dynamic reference current. This current does
not change the converter offset, but it may degrade the
gain and INL performance. The effect of this current can
be analyzed in two distinct situations.
For relatively small values of the external reference capaci-
tors (CREF < 1nF), the voltage on the sampling capacitor
settles almost completely and relatively large values for
the source impedance result in only small errors. Such
values for CREF will deteriorate the converter offset and
gain performance without signicant benets of reference
ltering and the user is advised to avoid them.
Larger values of reference capacitors (CREF > 1nF) may be
required as reference lters in certain congurations. Such
capacitors will average the reference sampling charge and
the external source resistance will see a quasi constant
reference differential impedance.
In the following discussion, it is assumed the input and
reference common mode are the same. Using internal
oscillator for 60Hz mode, the typical differential reference
resistanceis1MΩwhichgeneratesafull-scale(VREF/2)gain
error of 0.51ppm for each ohm of source resistance driving
the REF+ and REF– pins. For 50Hz/60Hz mode, the related
difference resistance is 1.1MΩ and the resulting full-scale
error is 0.46ppm for each ohm of source resistance driving
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