Datasheet

LM146, LM346
www.ti.com SNOSBH5B –MAY 2004–REVISED SEPTEMBER 2004
APPLICATION HINTS
Avoid reversing the power supply polarity; the device will fail.
COMMON-MODE INPUT VOLTAGE
The negative common-mode voltage limit is one diode drop above the negative supply voltage. Exceeding this
limit on either input will result in an output phase reversal. The positive common-mode limit is typically 1V below
the positive supply voltage. No output phase reversal will occur if this limit is exceeded by either input.
OUTPUT VOLTAGE SWING VS I
SET
For a desired output voltage swing the value of the minimum load depends on the positive and negative output
current capability of the op amp. The maximum available positive output current, (I
CL+
), of the device increases
with I
SET
whereas the negative output current (I
CL−
) is independent of I
SET
. Figure 26 illustrates the above.
Figure 26. Output Current Limit vs I
SET
INPUT CAPACITANCE
The input capacitance, C
IN
, of the LM146 is approximately 2 pF; any stray capacitance, C
S
, (due to external
circuit circuit layout) will add to C
IN
. When resistive or active feedback is applied, an additional pole is added to
the open loop frequency response of the device. For instance with resistive feedback (Figure 27), this pole
occurs at ½π (R1||R2) (C
IN
+ C
S
). Make sure that this pole occurs at least 2 octaves beyond the expected −3 dB
frequency corner of the closed loop gain of the amplifier; if not, place a lead capacitor in the feedback such that
the time constant of this capacitor and the resistance it parallels is equal to the R
I
(C
S
+ C
IN
), where R
I
is the input
resistance of the circuit.
Figure 27. Resistive Feedback Circuit Example
TEMPERATURE EFFECT ON THE GBW
The GBW (gain bandwidth product), of the LM146 is directly proportional to I
SET
and inversely proportional to the
absolute temperature. When using resistors to set the bias current, I
SET
, of the device, the GBW product will
decrease with increasing temperature. Compensation can be provided by creating an I
SET
current directly
proportional to temperature (see Typical Applications).
ISOLATION BETWEEN AMPLIFIERS
The LM146 die is isothermally layed out such that crosstalk between all 4 amplifiers is in excess of −105 dB
(DC). Optimum isolation (better than −110 dB) occurs between amplifiers A and D, B and C; that is, if amplifier A
dissipates power on its output stage, amplifier D is the one which will be affected the least, and vice versa. Same
argument holds for amplifiers B and C.
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