R

50

0

W

Z

50

0

W

ADS548x

INP

INM

S0176-04

R

200 W

ACSignal

Source

n=2:1

Dither

External Voltage Reference

ADS5481

ADS5482, ADS5483

The ADS548x performs optimally when the analog inputs are driven differentially. The circuit in Figure 33 shows

one possible configuration using an RF transformer with termination either on the primary or on the secondary of

the transformer. If voltage gain is required, a step-up transformer can be used.

Figure 33. Converting a Single-Ended Input to a Differential Signal Using an RF Transformer

The ADS548x family of devices contain a dither option that is enabled via the DITHEREN pin. Dither is a

technique applied to convert small static errors in the converter to dynamic errors, which will look similar to white

noise in the output. In virtually all cases tested, the harmonic performance is equal or better when dither is

enabled versus disabled. It improves the harmonics that are a function of the static errors. The dither is very low

level and will only be indicated in the output waveform as wideband noise that may slightly degrade the SNR

(<0.5dB). It is recommended that it be enabled, but users should allow the capability to disable it in the event

they suspect it may be degrading their specific application, or to compare the results during their evaluation.

Figure 19 through Figure 21 show the minor differences of dither ON/OFF when carefully studied.

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 = 1.2 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 ~1 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.

For dc-coupled applications that use the VCM pin of the ADS548x as the common mode of the signal in the

analog signal gain path prior to the ADC inputs, Figure 38 indicates very little change in VCM output as VREF is

externally adjusted.

The method for disabling the internal reference for use with an external reference is described in Table 4 .

Copyright © 2008, Texas Instruments Incorporated

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Product Folder Link(s): ADS5481 ADS5482, ADS5483