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AD7401AYRWZ-RL

Isolated Sigma-Delta Modulator

Analog Devices 

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 24. A signal

source driving the analog input must be able to provide the

charge onto the sampling capacitors every half MCLKIN cycle

and settle to the required accuracy within the next half cycle.

φA

1kΩ

VIN+

φB

2pF

φA

2pF

1kΩ

VIN–

φB

MCLKIN φA φB φA φB

Figure 24. Analog Input Equivalent Circuit

Because the AD7401A 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

AD7401A.

When a capacitive load is switched onto the output of an op

amp, the amplitude momentarily drops. 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 AD7401A. 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 25. 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 Ω and 47 pF, respectively.

R

VIN+

C AD7401A

R

VIN–

Figure 25. Differential Input RC Network

AD7401A

CURRENT SENSING APPLICATIONS

The AD7401A 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 AD7401A. The AD7401A

provides isolation between the analog input from the current

sensing resistor and the digital outputs. By selecting the appro-

priate shunt resistor value, a variety of current ranges can be

monitored.

Choosing RSHUNT

The shunt resistor values used in conjunction with the AD7401A

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 utilize the full input range of the

ADC, thus achieving maximum SNR performance.

When the peak sense current is known, the voltage range of the

AD7401A (±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

and 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.

RSHUNT 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 ter-

minals of the AD401A in excess of the absolute maximum

ratings. If ISENSE has a large high frequency component, take

care to choose a resistor with low inductance.

VOLTAGE SENSING APPLICATIONS

The AD7401A 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 moni-

tored exceeds the specified analog input range of the AD7401A,

a voltage divider network can be used to reduce the voltage to

be monitored to the required range.

Rev. 0 | Page 15 of 20

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