LM346 datasheet(9/24 Pages) NSC

9 / 24 page

LM146, LM346

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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 ISET

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, (ICL+), of the device increases

with ISET whereas the negative output current (ICL−) is independent of ISET. Figure 26 illustrates the above.

Figure 26. Output Current Limit vs ISET

INPUT CAPACITANCE

The input capacitance, CIN, of the LM146 is approximately 2 pF; any stray capacitance, CS, (due to external

circuit circuit layout) will add to CIN. 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 ½

π (R1||R2) (CIN + CS). 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 RI(CS + CIN), where RI 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 ISET and inversely proportional to the

absolute temperature. When using resistors to set the bias current, ISET, of the device, the GBW product will

decrease with increasing temperature. Compensation can be provided by creating an ISET 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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