# External Filters for the Analog Inputs of Metering ICs

Abstract: This application note examines ways to reduce adverse side effects, such as crosstalk and inaccuracies, in the Maxim portfolio of Teridian™ energy metering ICs. After briefly reviewing the ADC inputs, this article discusses the effect of different capacitors and resistors on metering ICs, as well as the use of ferrite beads to reduce RF susceptibility.

## Introduction

## Basic ADC Input Concepts

**Figure 1**shows an ideal simplified equivalent circuit of this network when the multiplexed switches are closed on one input. There are two clocks (θ1 and θ2) driving the switches (P1 and P2) that are 180 degrees out of phase.

*Figure 1. Simplified equivalent circuit of a switched capacitor input network.*

_{IN}. The converse occurs when P2 opens and P1 closes, namely the capacitance charge is transferred to the FIR filter represented as V

_{OUT}. Because the frequency of the clocks is approximately 5MHz, the transfer ideally should occur within approximately 2/(5MHz). Maxim recommends using a 1000pF capacitor at the ADC inputs to act as a "charge" reservoir to facilitate this transfer. In addition, Maxim recommends using a series resistor on the phase current ADC inputs to balance the 750Ω termination resistor on the divider string of the phase voltage resistor. Maxim uses 750Ω resistors in demo boards as illustrated in

**Figure 2**.

**Figure 3**shows the effects of the components on the switched capacitor equivalent circuit. It includes the impedance of the input multiplexer and P2 switches.

*Figure 3. Switched capacitor network with RC Input.*

(Eq. 1) |

**A lowpass filter, if needed or desired, can replace**the balance resistor and reservoir capacitor. The concepts of the switched capacitor network ADC input discussed above contribute to understanding the selection of this resistor and the capacitor values for ADC accuracy.

## Selecting the Type of Capacitor

**Figure 4**shows the effect of a 50Hz, small amplitude signal on both the NPO-type and X7R-type capacitors. The NPO-type capacitors that were tested have nearly identical responses, but the X7R-type capacitors show a voltage coefficient. This change of capacitance is a source of inaccuracy in Wh over phase as meter load current changes.

*Figure 4. The effect of a 50Hz amplitude signal on NPO-type and X7R-type capacitors.*

**Figure 5**shows this inaccuracy as a percent change of capacitance. To illustrate this effect, consider a typical 200A

_{RMS}meter using current transformers (CTs) that is calibrated at 30A

_{RMS}for magnitude and phase at room temperature. If the AC voltage from a CT sensor with a burden resistor at the current ADC input is 0.177V

_{RMS}at 200A

_{RMS}, then the AC voltage of a 200A

_{RMS}meter calibrated at 30A

_{RMS}is 0.027V

_{RMS}.The change of capacitance is approximately 1.5%. This difference becomes an uncompensated phase shift on the current channels and a source of inaccuracy in Wh readings when the power factor (PF) changes from 1.0 to 0.5.

*Figure 5. Inaccuracy due to capacitance drift.*

**Figure 6**shows that the X7R capacitance decreases over time. This decrease is caused by the relaxation or realignment of the electrical dipoles within the capacitor. The NPO-type capacitor does not experience this phenomenon.

*Figure 6. The effects of aging capacitors.*

## Selecting the Type of Resistor

## RF Rejection Filters

**Figure 7**shows the simplified equivalent circuit of the switched capacitor network, which includes a ferrite bead model.

*Figure 7. Simplified equivalent circuit of the switched capacitor network with ferrite bead.*

(Eq. 2) |

(Eq. 3) |

(Eq. 4) |

(Eq. 5) |

i(t) = D_{1}te^{-αt} + D_{2}e^{-αt} |
(Eq. 6) |

i(t) = B_{1}e^{-αt}(cosω_{d}t) + B_{1}e^{-αt}(sinω_{d}t) |
(Eq. 7) |

(Eq. 8) |

**Figure 8**shows the characteristics of the Murata BLM15HD102SN1D as an example. The L is approximately 1µH and R = 0.1Ω (use DC resistance for this discussion) in the 50Hz to 60Hz range.

*Figure 8. Example characteristics to calculate the dampening factor value.*

_{SW}= 50Ω, the damping factor is 0.08, i.e., under damped. Figure 9 shows the decaying oscillatory response of this under damped response in the bottom signal.

*Figure 9. Switched capacitor voltage with ferrite bead at ADC input.*

**Figure 9**, the single-ended ADC input is 0.25V. The red signal is P1. The blue signal is the voltage across the switched capacitor (C) with L = 1µH, R = 0.1Ω, R

_{SW}= 50Ω, and C = 10pF. The green signal is the voltage across the switched capacitor (C) with L = 2µH, R = 0.1Ω, R

_{SW}= 50Ω, and C = 10pF.

**Figure 10**).

*Figure 10. Recommended ferrite bead placement near ADC input.*

**Figure 11**shows the effect of this placement in the top signal. The response dampens quickly before P1 is closed. An alternative method would be to place a resistor between the ferrite bead and the ADC input and to change the damping factor.

*Figure 11. Switched capacitor voltage with ferrite bead and RC at ADC input.*