LTC6244
20
6244fb
Figure 8b: Output Noise Spectrum of the Circuit in Figure 8a.
Noise at 1MHz is 782nV/√Hz, Due Mostly to the Input Current
Noise Rising with Frequency
Total capacitance at the amplifi er’s input is now one C
CM
(2.1pF) plus the photodiode capacitance C
PD
(1.8pF), or
about 4pF accounting for parasitics. The shunt impedance
at 1MHz, for example, is X
C
= 1/(2πfC) = 39.8k, and
therefore, the noise gain at 1MHz is NG = 1+Rf/X
C
= 26.
The input voltage noise of this amplifi er is about 15nV/√Hz,
after accounting for the effects of R1 through R3, the
noise of the second stage and the fact that voltage noise
does rise with frequency. Multiplying the noise gain by
the input voltage noise gives an output noise density due
to voltage noise of 26 • 15nV/√Hz = 390nV/√Hz. But the
noise spectral density plot of Figure 8b shows an output
noise of 782nV/√Hz at 1MHz. The extra output noise is
due to input current noise, multiplied by the feedback
impedance. So while the circuit of Figure 8a does increase
bandwidth, it does not offer a noise advantage. Note,
however, that the 1.2mV
RMS
of noise is now measured in
a 2MHz bandwidth, instead of over a 350kHz bandwidth
of the previous example.
A Low Noise Fully Differential Buffer/Amplifi er
In differential signal conditioning circuits, there is often a
need to monitor a differential source without loading or
adding appreciable noise to the circuit. In addition, add-
ing gain to low level signals over appreciable bandwidth
is extremely useful. A typical application for a low noise,
high impedance, differential amplifi er is in the baseband
circuit of an RFID (radio frequency identifi cation) receiver.
The baseband signal of a UHF RFID receiver is typically a
low level differential signal at the output of a demodulator
with differential output impedance in the range of 100 to
400. The bandwidth of this signal is 1MHz or less.
The circuit of Figure 9a uses an LTC6244 to make a low
noise fully differential amplifi er. The amplifi er’s gain, input
impedance and –3dB bandwidth can be specifi ed indepen-
dently. Knowing the desired gain, input impedance and
–3dB bandwidth, R
G
, C
F
and C
IN
can be calculated from
the equations shown in Figure 9b. The common mode
gain of this amplifi er is equal to one (V
OUTCM
= V
INCM
)
and is independent of resistor matching. The component
values in the Figure 9a circuit implement a 970kHz, gain
= 5, differential amplifi er with 4k input impedance. The
output differential DC offset is typically less than 500µV.
The differential input referred noise voltage density is
shown in Figure 10. The total input referred noise in a
1MHz bandwidth is 16µV
RMS
.
APPLICATIONS INFORMATION
50k 1M
FREQUENCY (Hz)
6244 F08b
5M
OUTPUT NOISE (150nV/√Hz/DIV)
