AFE4403 datasheet(85/97 Pages) TI

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Duty Cycle (%)

Output voltage = 0 %FS

Output voltage = 10 %FS

Output voltage = 25 %FS

Output voltage = 50 %FS

Output voltage = 75 %FS

C006

100

102

104

106

Duty Cycle (%)

Output voltage = 0 %FS

Output voltage = 10 %FS

Output voltage = 25 %FS

Output voltage = 50 %FS

Output voltage = 75 %FS

C007

AFE4403

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SBAS650B – MAY 2014 – REVISED JULY 2014

Typical Application (continued)

Figure 128 shows the SNR of the signal chain as a function of the output voltage level. The data are taken by

looping back the transmitter outputs to the receiver inputs using an external op amp that converts the transmitter

voltage to a receiver input current. The loopback op amp and external resistors are an extra source of noise in

this measurement, so the actual noise levels are higher than the total noise of the transmitter plus the receiver.

The SNR in this curve (and other curves) is expressed in terms of dBFS, where the full-scale of the channel is

used as the reference level. Because the valid operating range of the signal chain is ±1 V, a full-scale of 2 V is

used for converting the output noise to a dBFS number. %FS refers to the percentage of the output level as a

function of the positive full-scale. For example, a 50 %FS curve corresponds to the case where the output level is

0.5 V. Also, the total noise in this curve is the total integrated noise in the digital output. All noise is contained in

the Nyquist band, which extends from –PRF / 2 to PRF / 2.

Figure 128. SNR over Nyquist Bandwidth vs Duty Cycle (Input Current with Tx-Rx Loopback)

Figure 129 is a representation of the same data as Figure 10. However, the noise is represented in terms of the

input-referred noise current in pArms. By multiplying this number with the TIA gain setting (500 k in this case),

the output noise voltage can be computed.

Figure 129. Input-Referred Noise Current over Nyquist Bandwidth vs Duty Cycle

(Input Current with Tx-Rx Loopback)

Figure 13 illustrates the SNR from the receiver as a function of the sampling duty cycle (which is the sampling

pulse duration referred to the pulse repetition period) for different settings of TIA gain. This curve is taken at 100-

Hz PRF. The maximum duty cycle is limited to 25%. A lower sampling duty cycle also means a lower LED pulse

duration duty cycle, which results in power saving.

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