AN1108 Advanced Linear Devices, AN1108 Datasheet - Page 8

• Manufacturer Part Number: AN1108
• Description: Application Note For Electrically Programmable Analog Device Applications
• Manufacturer: Advanced Linear Devices

HIGH OUTPUT IMPEDANCE CURRENT MIRROR
A calibrated cascode current mirror is shown in Figure 8A.
The input reference current can be set by means of a refer-
ence bias voltage and a fixed resistor by programming the
EPAD connected to the input reference current. The output
current I
the desired current mirror value. Although the mirrored cur-
rent can be matched to the input reference current, it can
also be set to a ratioed current value, where I
tional to the current source. The output impedance of this
programmable current mirror is approximately 20 Mohms.
The current mirror in Figure 8A can be easily modified to
become an electronically calibrated current source that
can be programmed and set at 68 A, as shown in Figure
8B. Note that a reference voltage V
voltage source is required. The tempco of R
factor that determines the tempco of the current source.
APPLICATION NOTE AN1108
V
bias
OUT
Current Source With EPAD Trimming
For low tempco set Ids
For V+ = +5V, R
GN1
of the current mirror can be set to within 1% of
V+
R
DN1
S12
1/4 ALD1108E
Ids
47K
Figure 7
68 A
+
-
ALD1701
V+
REF
or a relatively stable
GN1
REF
OUT
SN1
DN1
1/4 ALD1106
is the key
R
is propor-
Advanced Linear Devices
I
SET
O
V
REF
CURRENT
SOURCE
High Output Impedance Current Source
High Output Impedance Current Mirror
I
R
REF
REF
GN1
GN1
GN1
GN1
I
V-
V-
REF
I
REF
DN1
DN1
V-
Figure 8A
Figure 8B
V-
DN1
DN1
SN1
S12
S12
SN1
SN2
DN2
DN2
SN2
DN2
DN2
I
OUT
GN2
GN2
I
OUT
GN2
GN2
1/2 ALD1106
1/2 ALD1108E
1/2 ALD1106
1/2 ALD1108E
8