Conversion Boards:

In order to ease the evaluation of tiny packages such as the

micro SMD, there is a conversion board (LMC8101CONV)

available to board designers. This board converts a micro

SMD device into an 8 pin DIP package (see

Figure 2, Con-

version Board Pin out diagram) for easier handling and

evaluation. This board can be ordered from National Semi-

conductor by contacting http://www.national.com .

Increased Output Current:

Compared to the LMC7101, the LMC8101 has an improved

output stage capable of up to three times larger output

sourcing and sinking current. This improvement would allow

a larger output voltage swing range compared to the

LMC7101 when connected to relatively heavy loads. For

lower supply voltages this is an added benefit since it in-

creases the output swing range. For example, the LMC8101

can typically swing 2.5Vpp with 2mA sourcing and sinking

output current (Vs = 2.7V) whereas the LMC7101 output

swing would be limited to 1.9Vpp under the same conditions.

Also, compared to the LMC7101 in the SOT23 package, the

LMC8101 can dissipate more power because both the

MSOP and the micro SMD packages have 40% better heat

dissipation capability.

Lower 1/f noise:

The dominant input referred noise term for the LMC8101 is

the input noise voltage. Input noise current for this device is

of no practical significance unless the equivalent resistance

it looks into is 5M

Ω or higher.

The LMC8101’s low frequency noise is significantly lower

than that of the LMC7101. For example, at 10Hz, the input

referred spot noise voltage density is 85 nV

as com-

pared to about 200nV

for the LMC7101. Over a fre-

quency range of 0.1Hz to 100Hz, the total noise of the

LMC8101 will be approximately 60% less than that of the

LMC7101.

Lower THD:

When connected to heavier loads, the LMC8101 has lower

THD compared to the LMC7101. For example, with 5V sup-

ply at 10KHz and 2Vpp swing (Av = −2), the LMC8101 THD

(0.2%)is60% less than the LMC7101’s. The LMC8101 THD

can be kept below 0.1% with 3Vpp at the output for up to

10KHz (refer to the Typical Characteristics Plots).

Improving the Cap load drive capability:

This can be accomplished in several ways:

•

Output resistive loading increase:

The Phase Margin increases with increasing load (refer to

the Typical Characteristics Plots). When driving capacitive

loads, stability can generally be improved by allowing some

output current to flow through a load. For example, the cap

load drive capability can be increased from 8200pF to

16000pF if the output load is increased from 5K

Ω to 600Ω

(Av = +10, 25% overshoot limit, 10V supply).

•

Isolation resistor between output and cap load:

This resistor will isolate the feedback path (where excessive

phase shift due to output capacitance can cause instability)

from the capacitive load. With a 10V supply, a 100

Ω isolation

resistor allows unlimited capacitive load without oscillation

compared to only 300pF without this resistor (Av = +1).

•

Higher supply voltage:

Operating the LMC8101 at higher supply voltages allows

higher cap load tolerance. At 10V, the LMC8101’s low supply

voltage cap load limit of 300pF improves to about 600pF (Av

= +1).

•

Closed loop gain increase:

As with all Op Amps, the capacitive load tolerance of the

LMC8101 increases with increasing closed loop gain. In ap-

plications where the load is mostly capacitive and the resis-

tive loading is light, stability increases when the LMC8101 is

operated at a closed loop gain larger than +1.

DS101240-89

FIGURE 2. micro SMD Conversion Board pin-out

www.national.com

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