External Voltage Reference

ADS5463

For systems that require the analog signal gain to be adjusted or calibrated, this can be performed by using an

external reference. The dependency on the signal amplitude to the value of the external reference voltage is

characterized typically by Figure 34 (VREF = 2.4 V is normalized to 0 dB as this is the internal reference voltage.

This figure is the average gain adjustment from the data collected from -1dBFS to -6dBFS in 1 dB steps.) As can

be seen in the linear fit, this equates to approximately –0.3 dB of signal adjustment per 100 mV of reference

adjustment. The range of allowable variation depends on the analog input amplitude that is applied to the inputs

and the desired spectral performance, as can be seen in the performance versus external reference graphs in

Figure 35 and Figure 36. As the applied analog signal amplitude is reduced, more variation in the reference

voltage is allowed in the positive direction (which equates to a reduction in signal amplitude), whereas an

adjustment in reference voltage below the nominal 2.4 V (which equates to an increase in signal amplitude) is

not recommended below approximately 2.35 V. The power consumption versus reference voltage and operating

temperature should also be considered, especially at high ambient temperatures, because the lifetime of the

device is affected by internal junction temperature, see Figure 49.

The ADS5463 does not have a VCM output pin and primarily uses the VREF pin to provide the common-mode

voltage in dc-coupled applications. The ADS5463 (VCM = 2.4 V) and ADS5474 (VCM = 3.1 V) do not have the

same common-mode voltage, but they do share the same approximate VREF (2.4 V). To create a board layout

that may accommodate both devices in dc-coupled applications, route the VCM of the ADS5474 and the VREF

of the ADS5463 both to a common point that can be selected via a switch, jumper, or a 0

Ω resistor to be used

as the common-mode voltage of the driving circuit.

Copyright © 2006–2008, Texas Instruments Incorporated

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