LTC6240 datasheet(23/28 Pages) LINER

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LTC6240/LTC6241/LTC6242

624012fc

APPLICATIO S I FOR ATIO

1M Transimpedance Ampliﬁer with 43nV/√Hz

Output Noise

In a normal 1M transimpedance ampliﬁer, like that shown

on the back page of this data sheet, the output noise density

must be at least 130nV/√Hz at room temperature. This is

true even should the op amp be perfectly noiseless, because

the 1M resistor provides 130nV/√Hz of voltage noise at

room temperature independently of the op amp.

The circuit of Figure 13 provides an overall transimpedance

gain of 1MΩ, but it has an output noise density of only

43nV/√Hz, about 1/3 of the normal transimpedance ampli-

ﬁer. It does this by taking a higher initial transimpedance

gain of 10M and then attenuating by a factor of 10. The

transistor section provides voltage gain and works on a

54V supply voltage to guarantee adequate output swing.

By achieving an output swing of 50V before attenuation,

the circuit provides an output swing to 5V after attenu-

ation. The 10M resistor sets the gain of the TIA stage

and has a noise density of 400nV/√Hz. After attenuation,

the effective TIA gain drops to 1M while the noise ﬂoor

drops to 40nV/√Hz, which clearly dominates the observed

43nV/√Hz. Note the additional beneﬁt that the offset voltage

of the op amp is divided by 10. Worst case output offset

for this circuit is 150µV over temperature.

Reference Buffer

Figure 14 shows the LTC6240 being utilized as a buffer

in conjunction with the LT1019 reference. The passive

R-C ﬁlter attenuates the reference noise and the LTC6240

provides a low noise buffer, resulting in an output noise

of 8nV/√Hz.

54V

–5V

–1.5V

3pF

PHOTODIODE

6241 F13

–5V

10k

–

LTC6240HV

100pF

10M

0.3pF

10k

2.4k

33k

MPSA06

43k

9.09k

1/4W

VOUT

1M GAIN

(1V/

µA)

10M GAIN

(10V/

µA)

MPSA06

Figure 13. 1M Transimpedance Ampliﬁer with 43nV/√Hz Output Noise

Figure 14. Low Noise Reference Buffer

–5V

6241 F14

0.2

Ω

–

LTC6240HV

µF

CERAMIC

OR FILM

8nV/

√Hz

VOUT

µF

LT1019-2.5

180nV/

√Hz

LTC6240 Datasheet(PDF) 23 Page - Linear Technology

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