Circuit Description Pt - Heathkit IM-2215 Manual

Page 76
CIRCUIT DESCRIPTION
Refer to the Schematic Diagram, the Block Diagram (Illustration Booklet,
Page 18), and the following Pictorials as you read this "Circuit Descrip-
tion."
BASIC METER CIRCUIT
The analog-to-digital (A/D) converter, liquid crystal display (LCD), and DC
voltage reference make up the basic meter circuit, which provides a digital
indication for DC input voltages between — 199.9 millivolt and + 199.9
millivolt. The MOS/LSI integrated circuit (U4) contains all of the active
analog, digital, timing, and display drive circuits required for the 3-1/2
digit A/D conversion.
U1 is a temperature-compensated, 1.23 volt DC reference, which is used to
establish a reference voltage for the A/D conversion process. U1 also pro-
vides a known DC voltage at TP1 for calibration purposes. R8 sets the
reverse current through U1 at about 1 mA. Shunt capacitor C4 prevents U1
from oscillating; thus, ensuring reference stability under all operating
conditions. R11, R12, and R13 form a resistive voltage divider, which
establishes a 100.0 millivolt voltage drop across R11 when R13 is properly
adjusted.
The reference voltage across R11 is applied to A/D REF HI and REF LO
inputs in the voltaage and current modes of operation. This voltage charges
reference capacitor C14 once every conversion cycle (once every .4 second).
R24 and C14 provide low-pass filtering of the reference voltage, reducing
noise effect. When the Multimeter is operated in the resistance mode, the
reference circuit is used as a voltage source for resistance measurements
(see 'Resistance Measurements"').
C15 and R38 are oscillator components which determine the A/D conver-
ter's oscillator frequency of approximately 40 kHz. This frequency provides
2-1/2 conversions per second and helps suppress 50 and 60 Hz noise. The
entire conversion timing sequence is controlled by this oscillator.
U4 is powered by a 9-volt battery, which is connected across Vp), and Vs.
An internal regulator on U4 sets V,) (pin 1) at approximately + 2.8 volts
with respect to the COM pin (32). The COM pin is used as analog ground.
The TEST pin (37) is about —2.2 volts with respect to COM and is used as
the digital ground for U4 and the decimal and annunciator drives circuitry.
IN LO (pin 30) and IN HI (pin 31) are the analog signal inputs to the A/D
converter. R35 and C16 provide low-pass filtering of the input signal to
suppress AC line and noise signals from the input DC signal. Transistors Q4
and Q5 are connected as clamp diodes to protect U4 against excessive
positive or negative input voltages. Under overload conditions, R35 limits
the current passing through Q4 and Q5, while R36 limits the current to U4.
C13 is charged by internally generated voltages to compensate for offset
voltages in the analog circuitry of U4. This "automatic zeroing" ensures
that the digital display will be zero for a zero voltage input to the A/D
converter input.