External Voltage Reference

ADS5463

ADS54RF63

www.ti.com ................................................................................................................................................... SLAS515E – NOVEMBER 2006 – REVISED JULY 2009

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 53 (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 54 and Figure 55. 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 68.

The ADS5463/ADS54RF63 does not have a VCM output pin and uses the VREF pin to provide the

common-mode voltage in dc-coupled applications. The ADS5463/ADS54RF63 (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/ADS54RF63 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.

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