AD7294 datasheet(24/48 Pages) AD

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Data Sheet

AD7294

Rev. I | Page 23 of 47

gain. Therefore, for an input voltage of ±200 mV at the pins, an

output span of ±2.5 V is generated.

The current sensors on the AD7294 are designed to remove

any flicker noise and offset present in the sensed signal. This is

achieved by implementing a chopping technique that is transpa-

rent to the user. The VSENSE signal is first converted by the AD7294,

the analog inputs to the amplifiers are then swapped, and the

differential voltage is once again converted by the AD7294. The

two conversion results enable the digital removal of any offset

or noise. Switches on the amplifier inputs enable this chopping

technique to be implemented. This process requires 6 μs in total

to return a final result.

Choosing RSENSE

The resistor values used in conjunction with the current sense

amplifiers on the AD7294 are determined by the specific appli-

cation requirements in terms of voltage, current, and power.

Small resistors minimize power dissipation, have low inductance

to prevent any induced voltage spikes, and have good tolerance,

which 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 sense current is known, the voltage range of the

AD7294 current sensor (200 mV) is divided by the maximum

sense current to yield a suitable shunt value. If the power dissi-

pation 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 conversion results,

which are 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 losses. If the power dissi-

pation 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

AD7294 in excess of the absolute maximum ratings. Additional

protection is afforded to the current sensors on the AD7294 by

the recommended current limiting resistors, RF1 and RF2, as

illustrated in Figure 43. The AD7294 can handle a maximum

continuous current of 30 mA; thus, an RF2 of 1 kΩ provides

adequate protection for the AD7294.

If ISENSE has a large high frequency component, take care to

choose a resistor with low inductance. Low inductance metal

film resistors are best suited for these applications.

Current Sense Filtering

In some applications, it may be desirable to use external

filtering to reduce the input bandwidth of the amplifier (see

Figure 43). The −3 dB differential bandwidth of this filter is

equal to

BWDM = 1/(4πRC)

Note that the maximum series resistance on the RS(+) and

RS(−) inputs (as shown in Figure 42) is limited to a maximum

of 1 kΩ due to back-to-back ESD protection diodes from RS(+)

and RS(−) to VPP. Also, note that if RF1 and RF2 are in series

with R1 and R2 (shown in Figure 42), it affects the gain of the

amplifier. Any mismatch between RF1 and RF2 can introduce

offset error.

RF1

RF2

AD7294

VPP

RSENSE

ILOAD

RSx(–)

RSx(+)

10nF

VPP

Figure 43. Current Sense Filtering (RSX Can Be Either RS1 or RS2)

For certain RF applications, the optimum value for RF1 and

RF2 is 1 kΩ whereas CF1 can range from 1 μF to 10 μF. CF2 is a

decoupling capacitor for the VPP supply. Its value is application

dependent, but for initial evaluation, values in the range of 1 nF

to 100 nF are recommended.

Kelvin Sense Resistor Connection

When using a low value sense resistor for high current

measurement, the problem of parasitic series resistance can

arise. The lead resistance can be a substantial fraction of the

rated resistance, making the total resistance a function of lead

length. Avoid this problem by using a Kelvin sense connection.

This type of connection separates the current path through the

resistor and the voltage drop across the resistor. Figure 44 shows

the correct way to connect the sense resistor between the RS(+)

and RS(−) pins of the AD7294.

SENSE RESISTOR

CURRENT

FLOW FROM

SUPPLY

CURRENT

FLOW TO

LOAD

KELVIN

SENSE

TRACES

AD7294

RSX(+)

RSX(–)

Figure 44. Kelvin Sense Connections (RSX Can Be Either RS1 or RS2)

ANALOG COMPARATOR LOOP

The AD7294 contains two setpoint comparators that are used

for independent analog control. This circuitry enables users

to quickly detect if the sensed voltage across the shunt has

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