Datasheet
BASIC CONNECTIONS
QUIESCENT CURRENT CONTROL PIN
R
1
1.25k
Ω
TLV2262
OPA860
1/2 REF200
100
µ
A
R
2
425
Ω
V+
I
Q
Adjust
1 I
1
OPA860
SBOS331C – JUNE 2005 – REVISED AUGUST 2008 .......................................................................................................................................................
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It is also possible to vary the quiescent current with a
control signal. The control loop in Figure 45 shows
Figure 46 shows basic connections required for
1/2 of a REF200 current source used to develop
operation. These connections are not shown in
100mV on R
1
. The loop forces 125mV to appear on
subsequent circuit diagrams. Power-supply bypass
R
2
. Total quiescent current of the OPA860 is
capacitors should be located as close as possible to
approximately 37 × I
1
, where I
1
is the current made to
the device pins. Solid tantalum capacitors are
flow out of pin 1.
generally best.
The quiescent current of the transconductance
portion of the OPA860 is set with a resistor, R
ADJ
,
connected from pin 1 to – V
S
. It affects only the
operating currents of OTA sections. The bias circuitry
of the Buffer section is independent of the bias
circuitry for the OTA section; therefore, the quiescent
current cannot go below 5.8mA. The maximum
quiescent current is 12.7mA. R
ADJ
should be set
between 50 Ω and 1k Ω for optimal performance of the
OTA section. This range corresponds to the 12.5mA
quiescent current for R
ADJ
= 50 Ω , and 9mA for R
ADJ
=
1k Ω . If the I
Q
adjust pin is connected to the negative
supply, the quiescent current will be set by the 250 Ω
internal resistor.
Figure 45. Optional Control Loop for Setting
Quiescent Current
Reducing or increasing the quiescent current for the
OTA section controls the bandwidth and AC behavior
as well as the transconductance. With R
ADJ
= 250 Ω ,
this sets approximately 11.2mA total quiescent
current at 25 ° C. It may be appropriate in some
applications to trim this resistor to achieve the desired
quiescent current or AC performance.
Applications circuits generally do not show the
resistor R
Q
, but it is required for proper operation.
With a fixed R
ADJ
resistor, quiescent current
increases with temperature (see Figure 43 in the
Typical Characteristics section). This variation of
current with temperature holds the transconductance,
g
m
, of the OTA relatively constant with temperature
(another advantage over a transistor).
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