Avago Technologies ACPL-224-500E - Manual (Page 61)

AV02-4387EN 58 Avago Technologies
Low Cost Isolation Amplifier for Motor Speed and Position Measurement
Motor Control Applications
Description
This is a high-speed, low-cost isola-
tion amplifier that can be used for
the measurement of motor speed
and position. The analog signal
coming from the motor is assumed
to be 0 to 10 V, or 4 to 20 mA. This
circuit can be used in applications
where high bandwidth, low-cost,
and stable gain are required, but
where accuracy is not critical. This
circuit is a good example of how
a designer can trade off accuracy
to achieve improvements in band-
width and cost. The circuit has a
bandwidth of about 1.5 MHz with
stable gain characteristics and re-
quires few external components. R1
is selected to achieve an LED current
of about 7 - 10 mA at the nominal
input operating voltage according
to the following equation:
I
F
= (V
IN
/ R1) / K1
where K1 (i.e., I
PD1
/I
F
) of the
optocoupler is typically about 0.5%.
R2 is then selected to achieve the
desired output voltage according to
the equation, V
OUT
/V
IN
= R2 / R1. The
purpose of R4 and R6 is to improve
the dynamic response (i.e., stability)
of the input and output circuits by
lowering the local loop gains. R3 and
R5 are selected to provide enough
current to drive the bases of Q2 and
Q4. And R7 is selected so that Q4 op-
erates at about the same collector
current as Q2.
Performance of Circuit
• 1.5 MHz bandwidth
• Stable gain
• Low­cost support circuit
• Circuit couples only positive volt-
age signals
Benefits
• Low cost solution for coupling
positive voltage analog signals
• Simple way for isolating motor
speed and position analog sig-
nals
V IN
Q2 2N3904
Q1 2N3906
LED
ISOLATION
BARRIER
V CC1 +5 V
PD1
HCNR200/1
4-20 mA
R1
68 k Ω
R3
10 k Ω
R4
10 Ω
V OUT
Q4 2N3904
Q3 2N3906
V CC2 +5 V
PD2
R2
68 k Ω
R5
10 k Ω
R7
470
R6
10 Ω
M
A
0-10 V
+
–