LM1818 datasheet(8/14 Pages) NSC

8 / 14 page

Application Hints (Continued)

TLH7894 – 9

FIGURE 7 Auto Level-Meter Circuit

METER DRIVINGMOTIONAL PEAK LEVEL

RESPONSE

(Figure 7)

The meter drive output (pin 8) is capable of supplying 1 – 2

mA at a filtered DC voltage that is typically equal to 10 times

the RMS value of the signal applied to the ALC-meter drive

input (pin 4) The RC network connected to pin 7 of the IC

determines the memory constant of the meter circuit It is

therefore possible to store the peak input signal by giving

this RC network a long time constant or read the instanta-

neous signal level by giving this RC network a very short

time constant (ie no capacitor) This memory capacitor is

discharged within the integrated circuit at a discharge rate

related to the DC level on the meter output pin When the

meter output pin is between 0 VDC and 07 VDC there is a

50 mA discharge current when the pin is between 07V and

11V there is no internal discharge current and when the

voltage on pin 8 is greater than 11V there is a discharge

equivalent to a 33k resistor across the memory capacitor

These different discharge rates allow the meter circuit to

display fast accurate responses on the lower portion of the

meter display slow responses in the higher portion of the

meter display and rapid discharge when the voltage is

above the maximum reading the meter can display The re-

sistor in series with the meter can be adjusted such that the

previously mentioned responses coincide with the proper

points (0 VU and a3 VU) on the meter scale

LM1818 bảng dữ liệu(PDF) 8 Page - National Semiconductor (TI)