FS7755

Rev. 1.1

5/22

9

RESET

Reset Pin for the FS7755. A logic low on this pin will hold the ADCs and digital circuitry

in a reset condition. Bringing this pin logic low will clear the FS7755 internal registers.

10

This pin provides access to the on-chip voltage reference. The on-chip reference has a

nominal value of 2.5V ± 8% and a typical temperature coefficient of 30ppm/℃. An

external reference source may also be connected at this pin. In either case, this pin

should be decoupled to AGND with a 10μF ceramic capacitor and 100nF ceramic

capacitor.

11

AGND

This provides the ground reference for the analog circuitry in the FS7755, i.e., ADCs

and reference. This pin should be tied to the analog ground plane of the PCB. The

analog ground plane is the ground reference for all analog circuitry, e.g., anti-aliasing

filters and current and voltage transducers. For good noise suppression, the analog

ground plane should only connect to the digital ground plane at one point. A star ground

configuration will help to keep noisy digital currents away from the analog circuits.

12

SCF

Select Calibration Frequency. This logic input is used to select the frequency on the

calibration output CF. Table Ⅳ shows how the calibration frequencies are selected.

13, 14

S1, S0

These logic inputs are used to select one of four possible frequencies for the Power to

Impulse Converter. This offers the designer greater flexibility when designing the energy

meter. See Selecting a Frequency for an Energy Meter Application section.

15, 16

G1, G0

These logic inputs are used to select one of four possible gains for Channel 1, i.e., V1.

The possible gains are 1, 8, 16 and 32. See Analog Input section.

17

CLKIN

An external clock can be provided at this logic input. Alternatively, a parallel resonant

crystal can be connected across CLKIN and CLKOUT to provide a clock source for the

FS7755. The clock frequency for specified operation is 3.579545MHz. Crystal load

capacitance of between 22pF and 33pF (ceramic) should be used with the gate

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18

CLKOUT

A crystal can be connected across this pin and CLKIN as described above to provide a

clock source for the FS7755. The CLKOUT Pin can drive one CMOS load when an

external clock is supplied at CLKIN or by the gate oscillator circuit.

20

REVP

This logic output will go logic high when negative power is detected, i.e., when the

phase angle between the voltage and current signals is greater than 900. This output is

not latched and will be reset when positive power is once again detected. The output will

go high or low at the same time as a impulse is issued on CF.

21

DGND

This provides the ground reference for the digital circuitry in the FS7755, i.e., DSP. This

pin should be tied to the digital ground plane of the PCB. The digital ground plane is the

ground reference for all digital circuitry, e.g., counters (mechanical and digital), MCUs,

and indicator LEDs or LCDs. For good noise suppression, the analog ground plane

hld

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22

CF

Calibration Frequency Logic Output. The CF logic output gives instantaneous real

power information. This output is intended to be used for calibration purposes. Also see

SCF Pin description.

23, 24

F2, F1

Low Frequency Logic Outputs. F1 and F2 supply average real power information. The

logic outputs can be used to directly drive electromechanical counters and two-phase

stepper motors. See Transfer Function section.