Datasheet

LTC6240/LTC6241/LTC6242
25
624012fe
1M Transimpedance Amplifi er with 43nV/√Hz
Output Noise
In a normal 1M transimpedance amplifi er, 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 1M, but it has an output noise density of only
43nV/√Hz, about 1/3 of the normal transimpedance ampli-
fi er. 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 fl oor
drops to 40nV/√Hz, which clearly dominates the observed
43nV/√Hz. Note the additional benefi t 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 fi lter attenuates the reference noise and the LTC6240
provides a low noise buffer, resulting in an output noise
of 8nV/√Hz.
Figure 13. 1M Transimpedance Amplifi er with 43nV/√Hz Output Noise
Figure 14. Low Noise Reference Buffer
APPLICATIONS INFORMATION
5V
54V
–5V
–1.5V
3pF
PHOTODIODE
6241 F13
–5V
10k
–
+
LTC6240HV
100pF
10M
1%
0.3pF
1k
10k 2.4k
33k
MPSA06
43k
9.09k
1%
1/4W
1k
1%
V
OUT
1M GAIN
(1V/µA)
10M GAIN
(10V/µA)
MPSA06
5V
–5V
6241 F14
0.2Ω
–
+
LTC6240HV
1M
10µF
CERAMIC
OR FILM
8nV/√Hz
V
OUT
1µF
LT1019-2.5
180nV/√Hz