ADE7753ARSRL datasheet(23/60 Pages) AD

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ADE7753

Rev. C | Page 23 of 60

Antialias Filter

ADE7753 Reference Circuit

Figure 50 shows 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

ADE7753. 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 ½ and ¼ of the nominal value by using an

internal resistor divider, as shown in Figure 50.

Figure 47 also shows an analog low-pass filter (RC) on the input

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

Aliasing is an artifact of all sampled systems. Aliasing means

that frequency components in the input signal to the ADC,

which are higher than half the sampling rate of the ADC,

appear in the sampled signal at a frequency below half the

sampling rate. Figure 49 illustrates the effect. Frequency

components (arrows shown in black) above half the sampling

frequency (also know as the Nyquist frequency, i.e., 447 kHz)

are imaged or folded back down below 447 kHz. This happens

with all ADCs regardless of the architecture. In the example

shown, only frequencies near the sampling frequency, i.e.,

894 kHz, move into the band of interest for metering, i.e., 40 Hz

to 2 kHz. This allows the use of a very simple LPF (low-pass

filter) to attenuate high frequency (near 900 kHz) noise, and

prevents distortion in the band of interest. For conventional

current sensors, a simple RC filter (single-pole LPF) with a

corner frequency of 10 kHz produces an attenuation of

approximately 40 dB at 894 kHz—see Figure 49. The 20 dB per

decade attenuation is usually sufficient to eliminate the effects

of aliasing for conventional current sensors. However, for a

di/dt sensor such as a Rogowski coil, the sensor has a 20 dB per

decade gain. This neutralizes the –20 dB per decade attenuation

produced by one simple LPF. Therefore, when using a di/dt

sensor, care should be taken to offset the 20 dB per decade gain.

One simple approach is to cascade two RC filters to produce the

–40 dB per decade attenuation needed.

μA

PTAT

2.5V

1.7k

12.5k

REFIN/OUT

2.42V

MAXIMUM

LOAD = 10

μA

OUTPUT

IMPEDANCE

REFERENCE INPUT

TO ADC CHANNEL 1

(RANGE SELECT)

2.42V, 1.21V, 0.6V

02875-0-049

Figure 50. ADE7753 Reference Circuit Output

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

example, an external 2.5 V reference. Note that the nominal

reference value supplied to the ADCs is now 2.5 V, not 2.42 V,

which 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.

SAMPLING

FREQUENCY

IMAGE

FREQUENCIES

ALIASING EFFECTS

447

894

FREQUENCY (kHz)

02875-0-048

The voltage of the ADE7753 reference drifts slightly with

temperature—see the ADE7753 Specifications for the temperature

coefficient specification (in ppm/°C). The value of the temperature

drift varies from part to part. Since the reference is used for the

ADCs in both Channels 1 and 2, any x% drift in the reference

results in 2×% 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.

Alternatively, the meter can be calibrated at multiple temperatures.

Real-time compensation can be achieved easily by using the

on-chip temperature sensor.

Figure 49. ADC and Signal Processing in Channel 1 Outline Dimensions

ADC Transfer Function

The following expression relates the output of the LPF in the

Σ-Δ ADC to the analog input signal level. Both ADCs in the

ADE7753 are designed to produce the same output code for the

same input signal level.

CHANNEL 1 ADC

144

262

0492

)

(

OUT

ADC

Code

(1)

Figure 51 shows the ADC and signal processing chain for

Channel 1. In waveform sampling mode, the ADC outputs a

signed twos complement 24-bit data-word at a maximum of

27.9 kSPS (CLKIN/128). With the specified full-scale analog

input signal of 0.5 V (or 0.25 V or 0.125 V—see the Analog

Inputs section) the ADC produces an output code that is

approximately between 0x2851EC (+2,642,412d) and

0xD7AE14 (–2,642,412d)—see Figure 51.

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; this is equivalent to an input signal level of ±0.794 V.

However, for specified performance, it is recommended that the

full-scale input signal level of 0.5 V not be exceeded.

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