Tektronix DM505 Instruction Manual page 29

Section 3-DM
505
THEORY OF OPERATION
The basic circuitry of the DM
505
consists of an analog-
to-digital (AID) converter, display decoders and drivers,
a
clock, and power supplies (see the Block Diagram i n the
Diagrams section at the back of this manual). These
sections form a digital voltmeter that
measures
1.999 Vdc
full
scale.
A gain switch in the AID converter allows
measurement of 199.9
mV
full scale. An attenuator
enables measurement of 19.98, 199.9, and 1000 Vdc full
scale. Ac voltages are measured by passing the input
signal through a precision rectifier circuit that converts ac
voltages t o dc (an ac converter). Resistances are
measured by forcing
a
known current from
the ohms
converter through the unknown resistanceand measuring
the voltage across the unknown resistance. Low
resistance current shunts permit measurement of ac and
dc currents.
NOTE
In the following descriptions,
the
mond by
each
tjtle
refers
to the
circuit diagram in the Diagrams
manual.
numbered dia-
,
corraspondrng
section
of
this
Attenuator and Input Switching
@
With the
INPUT
EXT-INf pushbutton in the E X f
position, the input is measured at the front panel connec
tors. Resistances and voltages are measured between the
VOLTS/R and LOW connectors and current is measured
between the mA and LOW connectors. In t he I
NT
position,
switch S3 A , 8 selects the input at rear interface pins 288
(HI) and 28A (LO) for rear interface measurement of
resistances and voltages. Thef ront panel input impedance
is
10 MR
paralleled
by
less than 100 pF.
The VOLTS/R and LOW input connectors are con-
nected across R1312, a voltage divider. An
input measured
from thejunction of R13$2A and R1312B is attenuated by a
factor of 100. On the 1 kVdc scale, the input is measured
from the junction of R1312B and R1312Cand isattenuated
by a factor of 1000. Capacitors
C1312,
C1313, and C1311
compensate the attenuator for accurate ac signal
measurement. Capacitor GI313 is adjusted at
10
kHzfor
a
flat frequency response.
One solder-in shield and two additional shields at-
tached just inside the side covers shunt stray capacitance
across the attenuator to ground when measuring floating
ac voltages.
AC Converter
@
I n the VOLTS AC mode, dc input voltages are blocked
by capacitor C2114. The input of U1321 is protected
against excessive input voltages by R1424, CR1421, and
CR1422. Ac signals
are buttered by unity gain amplifier
U1321 and applied to the active rectifier circuit U1211,
which has high-speed diodes, CR1221 and CR1222, in the
feedback loop. Thiscircuit configuration eliminatesthe dc
voltage drop of the diodes. The half-wave rectified ac is
filtered by three-pole filter R1221, '21222, R1222, C1211,
Ri220, and C1221 to provide ripple-free dc voltage to the
A/D converter.
The active rectifier and filter respond to
the
average value of an ac signal, but the circuit gain is
calibrated to produce a dc output equivalent to the rms
value of a a 1 ne-wave input.
The basic gain of the ac converter is adjusted by R1322
for the 2 Yac range and R1323 fine-trims the gain for
200
mVac range. l ntegrated circuit U1211 has feed-
forward compensation provided by
C1212,
Ct
213,
and
R1216
t o obtain maximum bandwidth, Both the 20
Vac
and 200 mVac ranges use U1211 at again 10 times the gain
on the ot her ranges. The
500 Vac
range connects R1423 as
an additional attenuator to accommodate larger input
voltages. Dc feedback for U1211
is
provided by the
network consisting of R1223, R1224, and C1223. Dc offset
for U12t 1 is provided by R1321, the AC f ERO ADJ.
Ohms Converter
@
The ohms converter consists of precision current
source 0 1 31 1 and floating current mirrar U1311 (see Fig.
3-1). Transistor Q1311 generates a relatively low
temperature coefficient current, by mirroring the voltage
from Zener diode VR1211 across R1215 and R1213. The
current at the collector
of
Q1311 E is approximately i rnA
and is adjusted by R1213, the HI
fl
ADJ. This reference
current from Q1311 B appears at the inverting terminal of
U1311. The output of Q1311 sources current through
R1304 to stabilize the reference current. In the HI R
position, the output of U1311 is elevated 10
V
above its
input. In the LO
R
position, S2-H connects R1305 and
R1413 in parallel with R1304, lowering the output voltage
of U1311 and reducing thereferencecurrent to U1311 pin
2 by a factor of 10. Table 3-1 lists the values of source
current for the resistance ranges in the HI il: and LO R
positions. Resistor R1413 adjusts the LO reference
current to one-tenth of the
HI Cl reference current.