LTC6244HVCMS8 datasheet(19/24 Pages) LINER

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LTC6244

6244f

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 ampliﬁer’s input is now one CCM

(2.1pF) plus the photodiode capacitance CPD (1.8pF), or

about 4pF accounting for parasitics. The shunt impedance

at 1MHz, for example, is XC = 1/(2πfC) = 39.8kΩ, and

therefore, the noise gain at 1MHz is NG = 1+Rf/XC = 26.

The input voltage noise of this ampliﬁer 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 1.2mVRMS 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/Ampliﬁer

In differential signal conditioning circuits, there is often a

need to monitor a differential source without loading or

APPLICATIO S I FOR ATIO

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 ampliﬁer is in the baseband

circuit of an RFID (radio frequency identiﬁcation) 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

Ω to

400

Ω. The bandwidth of this signal is 1MHz or less.

The circuit of Figure 9a uses an LTC6244 to make a low

noise fully differential ampliﬁer. The ampliﬁer’s gain, input

impedance and –3dB bandwidth can be speciﬁed indepen-

dently. Knowing the desired gain, input impedance and

–3dB bandwidth, RG, CF and CIN can be calculated from

the equations shown in Figure 9b. The common mode

gain of this ampliﬁer is equal to one (VOUTCM = VINCM)

and is independent of resistor matching. The component

values in the Figure 9a circuit implement a 970kHz, gain

= 5, differential ampliﬁer 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 16µVRMS.

50k

FREQUENCY (Hz)

6244 F08b

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