Datasheet

DC ACCURACY AND OFFSET CONTROL THERMAL ANALYSIS
(NG=noninvertingsignalgainatdc)
(NG V ) (R I )
= (2 8mV) (375 1.1 A)
= 16.41mV
± ´
W m
OS(MAX) F OS(MAX)
+ ´
± ´ ´ ´
±
(10)
P =5V 19mA+4 5 /(4 (150 ||750 ))=295mWW
D
´ ´ ´ W
2
MaximumT =+85 C+(0.295W 95 C/W)=+113 C° ´ °
J
°
OPA4830
www.ti.com .................................................................................................................................................... SBOS350A – DECEMBER 2006 – REVISED MAY 2008
The balanced input stage of a wideband Maximum desired junction temperature sets the
voltage-feedback op amp allows good output dc maximum allowed internal power dissipation, as
accuracy in a wide variety of applications. The described below. In no case should the maximum
power-supply current trim for the OPA4830 gives junction temperature be allowed to exceed +150 ° C.
even tighter control than comparable products.
Operating junction temperature (T
J
) is given by T
A
+
Although the high-speed input stage does require
P
D
× θ
JA
. The total internal power dissipation (P
D
) is
relatively high input bias current (typically 5 µ A out of
the sum of quiescent power (P
DQ
) and additional
each input terminal), the close matching between
power dissipated in the output stage (P
DL
) to deliver
them may be used to reduce the output dc error
load power. Quiescent power is simply the specified
caused by this current. This reduction is achieved by
no-load supply current times the total supply voltage
matching the dc source resistances appearing at the
across the part. P
DL
depends on the required output
two inputs. Evaluating the configuration of Figure 74
signal and load; though, for resistive loads connected
(which has matched dc input resistances), using
to mid-supply (V
S
/2), P
DL
is at a maximum when the
worst-case +25 ° C input offset voltage and current
output is fixed at a voltage equal to V
S
/4 or 3V
S
/4.
specifications, gives a worst-case output offset
Under this condition, P
DL
= V
S
2
/(16 × R
L
), where R
L
voltage equal to Equation 10 :
includes feedback network loading.
Note that it is the power in the output stage, and not
into the load, that determines internal power
dissipation.
As a worst-case example, compute the maximum T
J
using an OPA4830 (TSSOP-14 package) in the circuit
of Figure 72 operating at the maximum specified
A fine-scale output offset null, or dc operating point
ambient temperature of +85 ° C and driving a 150 Ω
adjustment, is often required. Numerous techniques
load at mid-supply.
are available for introducing dc offset control into an
op amp circuit. Most of these techniques are based
on adding a dc current through the feedback resistor.
In selecting an offset trim method, one key
consideration is the impact on the desired signal path
frequency response. If the signal path is intended to
Although this value is still well below the specified
be noninverting, the offset control is best applied as
maximum junction temperature, system reliability
an inverting summing signal to avoid interaction with
considerations may require lower ensured junction
the signal source. If the signal path is intended to be
temperatures. The highest possible internal
inverting, applying the offset control to the
dissipation occurs if the load requires current to be
noninverting input may be considered. Bring the dc
forced into the output at high output voltages or
offsetting current into the inverting input node through
sourced from the output at low output voltages. This
resistor values that are much larger than the signal
puts a high current through a large internal voltage
path resistors. This configuration ensures that the
drop in the output transistors.
adjustment circuit has minimal effect on the loop gain
and therefore the frequency response.
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