ADE7756AN-REF datasheet(17/32 Pages) AD

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ADE7756

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Antialias Filter

Figure 21 also shows an analog low-pass filter (RC) on the input

to the modulator. This filter is present to prevent aliasing. Alias-

ing is an artifact of all sampled systems.

Figure 16 illustrates the effect, frequency components (arrows

shown in black) above half the sampling frequency (also known

as the Nyquist frequency, i.e., 447 kHz) is imaged or folded

back down below 447 kHz (arrows shown in grey). This will

happen with all ADCs no matter what the architecture is. In the

example shown it can be seen that only frequencies near the

sampling frequency, i.e., 894 kHz, will move into the band of

interest for metering, i.e., 40 Hz–2 kHz. This fact will allow us

to use a very simple LPF (Low-Pass Filter) to attenuate these

high frequencies (near 900 kHz) and so prevent distortion in the

band of interest. A simple RC filter (single-pole) with a corner

frequency of 10 kHz will produce an attenuation of approxi-

mately 40 dBs at 894 kHz—see Figure 15. This is sufficient to

eliminate the effects of aliasing.

FREQUENCY – kHz

ALIASING EFFECTS

447

894

IMAGE

FREQUENCIES

SAMPLING

FREQUENCY

Figure 16. ADC and Signal Processing in Channel 1

ADC Transfer Function

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

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

the ADE7756 are designed to produce the same output code for

the same input signal level.

Code ADC

REF

()

=×

1 512

262 144

Therefore, with a full-scale signal on the input of 1 V, and an

internal reference of 2.4 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 ±1.6 V.

However, for specified performance it is not recommended that

the full-scale input signal level of

±1 V be exceeded.

Reference Circuit

Shown in Figure 17 is a simplified version of the reference output

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

ADE7756. However, Channel 1 has three input range selections

that 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 17.

1.7k

PTAT

REFIN/OUT

MAXIMUM

LOAD = 10 A

2.42V

12.5k

REFERENCE INPUT TO ADC

CHANNEL 1 (RANGE SELECT)

2.42V, 1.21V, 0.6V

2.5V

OUTPUT IMPEDANCE

60 A

Figure 17. 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 1 V to 1.03 V.

The voltage of ADE7756 reference drifts slightly with temperature

—see ADE7756 Specifications for the temperature coefficient

specification (in ppm/

°C). The value of the temperature drift varies

from part to part. On A-grade parts, the maximum temperature

drift is not guaranteed. 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 accuracy. 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 external voltage reference or to use

B-grade parts. Alternatively, the meter can be calibrated at

multiple temperatures. Real-time compensation can be easily

achieved using the on the on-chip temperature sensor.

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