LTC6242IGN datasheet(15/24 Pages) LINER

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LTC6241/LTC6242

62412f

APPLICATIO S I FOR ATIO

Proprietary design techniques are used to obtain simul-

taneous low 1/f noise and low input capacitance. Low

input capacitance is important when the ampliﬁer is used

with high source and feedback resistors. High frequency

noise from the ampliﬁer tail current source, ITAIL in Fig-

ure 1, couples through the input capacitance and appears

across these large source and feedback resistors. As an

example, the photodiode ampliﬁer of Figure 11 on the last

page of this data sheet shows the noise results from the

LTC6241 and the results of a competitive CMOS ampliﬁer.

The LTC6241 output is the ideal noise of a 1MΩ resistor

at room temperature, 130nV√Hz.

Half the Noise

The circuit shown in Figure 3 can be used to achieve even

lower noise voltage. By paralleling 4 ampliﬁers the noise

voltage can be lowered by √4, or half as much noise. The

√ comes about from an RMS summing of uncorrelated

noise sources. This circuit maintains extremely high input

resistance, and has a 250Ω output resistance. For lower

output resistance, a buffer ampliﬁer can be added without

inﬂuencing the noise.

Stability

The good noise performance of these op amps can be at-

tributed to large input devices in the differential pair. Above

several hundred kilohertz, the input capacitance rises and

can cause ampliﬁer stability problems if left unchecked.

When the feedback around the op amp is resistive (RF), a

pole will be created with RF, the source resistance, source

capacitance (RS, CS), and the ampliﬁer input capacitance.

In low gain conﬁgurations and with RF and RS in even

the kilohm range (Figure 4), this pole can create excess

phase shift and possibly oscillation. A small capacitor CF

in parallel with RF eliminates this problem.

Low Noise Single-Ended Input to Differential Output

Ampliﬁer

The circuit on the ﬁrst page of the data sheet is a low noise

single-ended input to differential output ampliﬁer, with a

200k input impedance. The very low input bias current

of the LTC6241 allows for these large input and feedback

resistors. The 200k resistors, R1 and R2, along with C1

and C2 set the –3dB bandwidth to 80kHz. Capacitor C3 is

used to cancel effects of input capacitance, while C4 adds

Figure 3. Parallel Ampliﬁer Lowers Noise by 2x

Figure 4. Compensating Input Capacitance

–

CIN

6241 F04

OUTPUT

Ω

6241 F03

–

1/4

LTC6242

Ω

–

1/4

LTC6242

Ω

–

1/4

LTC6242

Ω

–

1/4

LTC6242

+2.5

–2.5

VIN

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