AD7294 datasheet(17/45 Pages) AD

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Preliminary Technical Data

AD7294

Rev. PrB | Page 17 of 45

CURRENT SENSOR

Two current-sense amplifiers are provided, which can

accurately amplify small differential current shunt voltages in

the presence of rapidly changing common mode voltages. Each

accepts a (VPP) ±200 mV full-scale input voltage and supplies a

(VCC/2) ± 2.5 V signal to the ADC.

The current sense in the AD7294 is a high-side current sense

amplifier, which allows for the monitoring of currents on the VDD

line ranging from AVDD up to 48 V, see Figure 18. The sense

amplifier works correctly for common mode voltages on RS(−)

and RS(+) of between 47 V (VPP − 1 V) and 48.2 V (VPP + 200

mV). It gives a full scale output to the ADC for a differential

input of ± 200 mV, e.g. 48 V on RS+ and 47.8 on RS−. An input

signal of greater than 240 mV magnitude should trigger an

alert. An alert will also be triggered if the common mode

voltages on RS+ and RS− go outside the specified limits, as the

amplifier will no longer be in its linear region. The inputs can

be sampled at approximately 6 μs intervals, giving a Nyquist

frequency of approximately 160 KHz.

AD7294

AVDD to +48V

RS(+)

RS(-)

RSENSE

ILOAD

VOUT

TO MUX

Figure 18. High-Side Current Sense

The AD7294 is comprised of two main blocks, a differential and

an instrumentation amplifier. A load current flowing through

the external shunt resistor produces a voltage at the input

terminals of the AD7294. Resistors R1 and R2 connect the input

terminals to the differential amplifier (A1). A1 nulls the voltage

appearing across its own input terminals by adjusting the

current through R1 and R2 with transistors Q1 and Q2. When

the input signal to the AD7294 is 0, the currents in R1 and R2

are equal. When the differential signal is nonzero, the current

increases through one of the resistors and decreases in the

other. The current difference is proportional to the size and

polarity of the input signal. Since the differential input voltage is

converted into a current, common-mode rejection is no longer

reliant on resistor matching, and high accuracy and

performance is provided throughout the wide common-mode

voltage range. The amplifier is chopper stabilized with a

switching frequency of approximately 300kHz.

The differential currents through QI and Q2 are converted into

a differential voltage due to R3 and R4. A2 is configured as an

instrumentation amplifier, and this differential input signal is

converted into a single-ended output voltage by A2. The gain is

internally set with thin-film resistors to 12.5V/V. Hence for an

input voltage of ± 200 mV an output span of ± 2.5 V will be

generated.

Note that when using the external reference for the ADC, the

maximum ISENSE input span is 240 mV with a gain of 12.5 in the

ISENSE amplifier. Therefore, the maximum usable span on the

ADC is 3 V. If an external reference of 5 V is required by the

user, only 60% the ADC span will be used for the ISENSE

conversion.

Choosing RSENSE

Example calculation:

The AD7294 current sense has a specified full-scale sense range

of ± 200 mV. With a VPP of, for example, 48V and a RLOAD of,

for example, 50 Ω, the ILOAD is :

ILOAD = VPP / RLOAD = 48 V / 50 Ω = 0.96 A

With a full scale sense range of 200 mV, sense resistor value is:

RSENSE = FSVSENSE / ILOAD = 200 mV / 0.96 A = 208.3 m Ω

In applications monitoring very high currents, RSENSE must be

able to dissipate the I2R losses. If the resistor’s rated power

dissipation is exceeded, its value may drift or it may fail

altogether, causing a differential voltage across the terminals in

excess of the absolute maximum ratings. If ISENSE has a large high

frequency component, care should be taken to choose a resistor

with low inductance. Wire-wound resistors have the highest

inductance, metal-film resistors are somewhat better, and low

inductance metal-film resistors are best suited for these

applications.

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. This problem can be avoided 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 19 shows the correct way to connect the sense resistor

between the VCC and SENSE pins of the AD7294.

SENSE RESISTOR

CURRENT FLOW

FROM SUPPLY

CURRENT FLOW

TO LOAD

KELVIN SENSE

TRACES

AD7294

RSX(+)

RSX(-)

Figure 19. Kelvin Sense Connections

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