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参数资料
| 型号： | EVAL-AD7400AEDZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 6/20页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL AD7400A |
| 标准包装： | 1 |
| 系列： | iCoupler® |
| ADC 的数量： | 1 |
| 位数： | 16 |
| 采样率（每秒）： | 10M |
| 数据接口： | 串行 |
| 输入范围： | ±320 mV |
| 在以下条件下的电源（标准）： | 95mW @ 10MSPS |
| 工作温度： | -40°C ~ 125°C |
| 已用 IC / 零件： | AD7400A |
| 已供物品： | 板 |
AD7400A
Data Sheet
Rev. D | Page 14 of 20
DIFFERENTIAL INPUTS
The analog input to the modulator is a switched capacitor
design. The analog signal is converted into charge by highly
linear sampling capacitors. A simplified equivalent circuit
diagram of the analog input is shown in Figure 22. A signal
source driving the analog input must be able to provide the
charge onto the sampling capacitors every half MCLKOUT cycle
and settle to the required accuracy within the next half cycle.
φA
φB
1k
VIN–
φA
φB
1k
VIN+
2pF
φA
MCLKOUT
07077-
027
Figure 22. Analog Input Equivalent Circuit
Because the AD7400A samples the differential voltage across its
analog inputs, low noise performance is attained with an input
circuit that provides low common-mode noise at each input.
The amplifiers used to drive the analog inputs play a critical
role in attaining the high performance available from the
AD7400A.
When a capacitive load is switched onto the output of an op
amp, the amplitude drops momentarily. The op amp tries to
correct the situation and, in the process, hits its slew rate limit.
This nonlinear response, which can cause excessive ringing, can
lead to distortion. To remedy the situation, a low-pass RC filter
can be connected between the amplifier and the input to the
AD7400A. The external capacitor at each input aids in supplying
the current spikes created during the sampling process, and the
resistor isolates the op amp from the transient nature of the load.
The recommended circuit configuration for driving the differential
inputs to achieve best performance is shown in Figure 23. A
capacitor between the two input pins sources or sinks charge
to allow most of the charge that is needed by one input to be
effectively supplied by the other input. The series resistor again
isolates any op amp from the current spikes created during the
sampling process. Recommended values for the resistors and
capacitor are 22 and 47 pF, respectively.
R
VIN–
R
VIN+
C
AD7400A
07077-
028
Figure 23. Differential Input RC Network
CURRENT SENSING APPLICATIONS
The AD7400A is ideally suited for current sensing applications
where the voltage across a shunt resistor is monitored. The load
current flowing through an external shunt resistor produces a
voltage at the input terminals of the AD7400A. The AD7400A
provides isolation between the analog input from the current
sensing resistor and the digital outputs. By selecting the appropriate
shunt resistor value, a variety of current ranges can be monitored.
Choosing RSENSE
The shunt resistor values used in conjunction with the AD7400A
are determined by the specific application requirements in terms of
voltage, current, and power. Small resistors minimize power
dissipation, while low inductance resistors prevent any induced
voltage spikes, and good tolerance devices reduce current
variations. The final values chosen are a compromise between
low power dissipation and good accuracy. Low value resistors
have less power dissipated in them, but higher value resistors
may be required to use the full input range of the ADC, thus
achieving maximum SNR performance.
When the peak sense current is known, the voltage range of the
AD7400A (±200 mV) is divided by the maximum sense current
to yield a suitable shunt value. If the power dissipation in the shunt
resistor is too large, the shunt resistor can be reduced, in which
case, less of the ADC input range is used. Using less of the ADC
input range results in performance that is more susceptible to noise
and offset errors because offset errors are fixed and are thus more
significant when smaller input ranges are used.
RSENSE must be able to dissipate the I2R power losses. If the power
dissipation rating of the resistor is exceeded, its value may drift
or the resistor may be damaged, resulting in an open circuit.
This can result in a differential voltage across the terminals of
the AD400A in excess of the absolute maximum ratings (see
care to choose a resistor with low inductance.
VOLTAGE SENSING APPLICATIONS
The AD7400A can also be used for isolated voltage monitoring.
For example, in motor control applications, it can be used to
sense bus voltage. In applications where the voltage being
monitored exceeds the specified analog input range of the
AD7400A, a voltage divider network can be used to reduce
the voltage being monitored to the required range.
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