External Voltage Reference

ExternalVREFApplied

V

-

1.0

0.5

0

-0.5

-1.0

-1.5

-3.0

2.2

2.3

2.4

2.5

3.1

2.6

2.7

2.8

f =400MSPS

f

=70MHz

A

=< 1dBFS

S

IN

IN

-

2.9

3.0

BestFit:

y= 3.14x+7.5063

-

-2.0

-2.5

ExternalVREFApplied

V

-

90

80

70

60

50

40

2.05 2.15 2.25 2.35 2.45

3.15

f =400MSPS

f

=70MHz

S

IN

2.55 2.65 2.75 2.85 2.95 3.05

A

= 6dBFS

-

IN

A

= 4dBFS

-

IN

A

= 3dBFS

-

IN

A

= 5dBFS

-

IN

A

= 2dBFS

-

IN

A

= 1dBFS

-

IN

ADS5474

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 35 (VREF = 2.4 V is normalized to 0 dB as this is the internal reference

voltage). 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 36 and Figure 37. 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 50.

For dc-coupled applications that use the VCM pin of the ADS5474 as the common mode of the signal in the

analog signal gain path prior to the ADC inputs, the information in Figure 39 is useful to consider versus the

allowable common-mode range of the device that is receiving the VCM voltage, such as an operational amplifier.

Because it is pin-compatible, it is important to note that the ADS5463 does not have a VCM 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 = 3.1V) do not have the same common-mode voltage. To create a board layout that may

accommodate both devices in dc-coupled applications, route VCM and VREF both to a common point that can

be selected via a switch, jumper, or a 0

Ω resistor.

Figure 35. Signal Gain Adjustment versus External

Reference (VREF)

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Product Folder Link(s): ADS5474