AD7293 datasheet(23/78 Pages) AD

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Data Sheet

AD7293

Rev. B | Page 23 of 78

The AD7293 high-side current sense amplifier is configured as

a differential integrator.

Figure 39. Current Sensor Internal Diagram

Before each measurement, the integrator is held in a reset state

for 9.2 µs. The input is then connected and measured for a

programmable amount of time, resulting in a gain equal to the

following:

Gain = Integration Time/(224 kΩ × 6 pF)

Table 14. Current Sensor Gain Settings

Code

Typical

Gain

Voltage Across

RSENSE1 (mV)

Typical

Integration

Time (µs)

0000 (Default)

6.25

±200

8.4

0001

12.5

±100

16.8

0010

18.75

±66.67

25.2

0011

±50

33.6

0100

37.5

±33.33

50.4

0101

±25

67.2

0110

±16.67

100.8

0111

100

±12.5

134.4

1000

200

±6.25

268.8

1001

400

±3.125

537.6

1010

781.25

±1.6

1050

RSENSE is the external sense resistor.

When integration is complete, the input switches open, keeping

the output constant until the ADC completes its conversion of

the output signal. If no other ADC channels are enabled, the

conversion takes an additional 4.2 µs. Otherwise, the current

sense amplifier waits for its turn in the ADC conversion

sequence before resetting and starting a new measurement.

Keep the external source impedance low with respect to the

input resistance of the integrator to avoid creating a gain error.

Calculate the current sensor input channel LSB as follows:

VSENSE LSB = (2 × REFADC)/(Gain × 4096)

VRSx+ − VRSx− = −REFADC/Gain, with DOUT = 0x000

VRSx+ − VRSx− = 0 V, with DOUT = 0x7FF

VRSx+ − VRSx− = REFADC/Gain, with DOUT = 0xFFF

where:

VSENSE LSB is the current sense input channel LSB size in volts.

VRSx+ is the voltage for the RSx+ pins.

VRSx− is the voltage for the RSx− pins.

REFADC = 1.25 V.

Gain can be set between 6.25 and 781.25 as shown in Table 14.

ISENSE LSB = 2 × (REFADC/(Gain × 4096 × RSENSE))

where:

ISENSE LSB is the current sense input channel LSB size in

amperes.

RSENSE is the external sense resistor.

Choosing the External Sense Resistor (RSENSE)

The resistor values used in conjunction with the current sense

amplifiers on the AD7293 are determined by the specific applica-

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

Small resistors minimize power dissipation, have low induct-

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

which reduces current variations. The final values chosen are a

compromise between low power dissipation and good accuracy.

Low value resistors have less power dissipation and good accu-

racy; however, higher value resistors may be required to use the

full input range of the ADC.

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

AD7293 current sensor is divided by the maximum sense

current to yield a suitable resistor value. If the power dissipation

in the shunt resistor is too large, the shunt resistor can be

reduced, in which case, the current sensor gain can be increased

to maximize the ADC input range used.

RSENSE must be able to dissipate the I2R 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. If

the power dissipation of the resistor is exceeded, it can result in

a differential voltage across the AD7293 terminals in excess of

the absolute maximum ratings.

)

(MAX

SENSE

Range

Voltage

Input

Sensor

Current

≤

where:

RSENSE is the value of the current sense resistor in Ω.

Current Sensor Input Voltage Range is the current sensor

amplifier input voltage range as dictated by the gain setting

chosen (see Table 14).

ISENSE (MAX) is the maximum current required in A.

TEMPERATURE SENSOR

The AD7293 contains one local and two remote temperature

sensors. The temperature sensors can continuously monitor the

three temperature inputs, and new readings are automatically

available every 5 ms.

The on-chip temperature sensor measures the device die

temperature. The internal temperature sensor measures

between −40°C and 125°C, where the LSB size is 0.125°C.

The AD7293 includes two remote temperate sensors. The

device is factory calibrated to work with 2N3906 discrete

transistors.

224kΩ

–

6pF

RESET

RSx+

RSx–

RESET, HOLD

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