ADE7759ARSRL datasheet(17/32 Pages) AD

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ADE7759

–17–

IMAGE

FREQUENCIES

SAMPLING

FREQUENCY

ALIASING EFFECTS

447

894

FREQUENCY – kHz

Figure 21. ADC and Signal Processing in Channel 1

For a di/dt sensor such as a Rogowski coil, however, the sensor

has 20 dB per decade gain. This will neutralize the –20 dB per

decade attenuation produced by this simple LPF and nullifies the

antialias filter. Therefore, when using a di/dt sensor, measure

should be taken to offset the 20 dB per decade gain coming from

the di/dt sensor and produce sufficient attenuation to eliminate

any aliasing effect. One simple approach is to cascade two RC

filters to produce –40 dB per decade attenuation. The transfer

function for a cascaded filter is the following:

sR C

s R C R C

()

1 12 21 2

1 1 2 2

where R1C1 represents the RC used in the first stage of the

cascade and R2C2 in that of the second stage. The s

2 term in the

transfer function produces a –40 dB/decade attenuation. Note

that to minimize the measurement error, especially at low power

factor, it is important to match the phase angle between the voltage

and the current channel. The small phase mismatch in the exter-

nal antialias filter can be corrected using the Phase Calibration

ADC Transfer Function

Below is an expression which relates the output of the LPF in

the sigma-delta ADC to the analog input signal level. Both

ADCs in the ADE7759 are designed to produce the same out-

put code for the same input signal level.

Code ADC

REF

()

=×

3 0492

262 144

Therefore, with a full-scale signal on the input of 0.5 V and an

internal reference of 2.42 V, the ADC output code is nominally

165,151 or 2851Fh. The maximum code from the ADC is

±262,144, which is equivalent to an input signal level of ±0.794 V.

However, for specified performance it is not recommended that

the full-scale input signal level of 0.5 V be exceeded.

Reference Circuit

Shown in Figure 22 is a simplified version of the reference out-

put circuitry. The nominal reference voltage at the REFIN/OUT

pin is 2.42 V. This is the reference voltage used for the ADCs in

the ADE7759. However, Channel 1 has three input range selec-

tions, which are selected by dividing down the reference value

used for the ADC in Channel 1. The reference value used for

Channel 1 is divided down to 1/2 and 1/4 of the nominal value

by using an internal resistor divider as shown in Figure 22.

60 A

PTAT

2.5V

1.7k

12.5k

REFIN/OUT

2.42V

MAXIMUM

LOAD = 10 A

OUTPUT

IMPEDANCE

REFERENCE INPUT

TO ADC CHANNEL 1

(RANGE SELECT)

2.42V, 1.21V, 0.6V

Figure 22. ADC and Reference Circuit Output

The REFIN/OUT pin can be overdriven by an external source,

e.g., an external 2.5 V reference. Note that the nominal refer-

ence value supplied to the ADCs is now 2.5 V not 2.42 V. This

has the effect of increasing the nominal analog input signal

range by 2.5/2.42

100% = 3%, or from 0.5 V to 0.5165 V.

The internal voltage reference on the ADE7759 has a tempera-

ture drift associated with it—see ADE7759 Specifications for the

temperature coefficient specification (in ppm

°C). The value of

the temperature drift varies slightly from part to part. Since the

reference is used for the ADCs in both Channel 1 and 2, any x%

drift in the reference will result in 2x% deviation of the meter

reading. The reference drift resulting from temperature changes

is usually very small and it is typically much smaller than the

drift of other components on a meter. However, if guaranteed

temperature performance is needed, one needs to use an exter-

nal voltage reference. Alternatively, the meter can be calibrated

at multiple temperatures. Real-time compensation can be achieved

easily using the on-chip temperature sensor.

CHANNEL 1 ADC

Figure 23 shows the ADC and signal processing chain for Chan-

nel 1. In waveform sampling mode the ADC outputs a signed

two’s complement 20-bit data word at a maximum of 27.9 kSPS

(CLKIN/128). The output of the ADC can be scaled by

±50%

to perform an overall power calibration or to calibrate the ADC

output. While the ADC outputs a 20-bit two’s complement value,

the maximum full-scale positive value from the ADC is limited

to 40,000h (+262,144 decimal). The maximum full-scale negative

value is limited to C0000h (–262,144 decimal). If the analog

inputs are over-ranged, the ADC output code will clamp at these

values. With the specified full-scale analog input signal of 0.5 V

(or 0.25 V or 0.125 V—see Analog Inputs section) the ADC will

produce an output code that is approximately 63% of its full-scale

value. This is illustrated in Figure 23. The diagram in Figure 23

shows a full-scale voltage signal being applied to the differential

inputs V1P and V1N. The ADC output swings between D7AE1h

(–165,151) and 2851Fh (+165,151). This is approximately 63%

of the full-scale value 40,000h (262,144). Over-ranging the

analog inputs with more than 0.5 V differential (0.25 or 0.125,

depending on Channel 1 full-scale selection) will cause the ADC

output to increase towards its full-scale value. However, for speci-

fied operation the differential signal on the analog inputs should

not exceed the recommended value of 0.5 V.

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