ADS5474_08 Datasheet, PDF (21/38 Pages) Texas Instruments – 14-Bit, 400-MSPS Analog-to-Digital Converter

ADS5474
www.ti.com......................................................................................................................................................... SLAS525A – JULY 2007 – REVISED AUGUST 2008
External Voltage Reference
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.
1.0
0.5
0
-0.5
f_S = 400 MSPS
f_IN = 70 MHz
A_IN = < -1 dBFS
Best Fit:
y = -3.14x + 7.5063
90
A_IN = -5 dBFS
80
A_IN = -4 dBFS
70
A_IN = -6 dBFS
-1.0
-1.5
-2.0
-2.5
-3.0
2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1
External VREF Applied - V
Figure 35. Signal Gain Adjustment versus External
Reference (VREF)
60
A_IN = -1 dBFS
50
A_IN = -3 dBFS
A_IN = -2 dBFS
f_S = 400 MSPS
f_IN = 70 MHz
40
2.05 2.15 2.25 2.35 2.45 2.55 2.65 2.75 2.85 2.95 3.05 3.15
External VREF Applied - V
Figure 36. SFDR versus External VREF and A_IN
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