Meter hardware

AN2288

12/29

4.1.5

Tamper detection block

Two analog switches and one control pin with one fast switching transistor is used to detect

tampering.

Figure 8.

Tamper detection circuit

4.1.6

Gain switching block

Gain multipliers are used to get the large load range with Ib=10A and Imax=55A. In this

application, there are four gain multipliers x2, x8, x32 and x128. A particular gain factor is

actively used by the microcontroller, depending on the signal strength. For the lowest signal

strength x128 gain factor is used. The operational amplifiers are used in non-inverting

mode.

The exact gain factors are:

When the voltage after x2 multiplier decreases below ~2.03Vp-p/4(=~0.508V), the gain

factor will change to x8 or when the voltage after x8 multiplier increases after ~2.03V, the

gain factor will change to x2. There is some hysteresis also implemented for this. So, when

upper half of sinewave after x2 decreases below ~0.508V/2 (= ~0.254V), the active gain

analog switches

Voltage out from phase CT

Voltage out from Neutral CT

5V

Control signal from MCU

To gain multiplier

Gain

1

R

F

R

1

-------

+

⎝⎠

⎛⎞

=

x2

1

470E

470E

--------------

+

⎝⎠

⎛⎞

2

==

x8

1

1.5K

220E

--------------

+

⎝⎠

⎛⎞

7.818

==

x32

1

16K 51K

||

390E

---------------------------

+

⎝⎠

⎛⎞

32.23

==

x128

1

51K

390E

--------------

+

⎝⎠

⎛⎞

131.77

==