ADS5400IPZP datasheet(23/55 Pages) TI1

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ADS5400

AIN

100 W

ACSignal

Source

1:1

Bipolar

Transistor

Buffer

AINP

AVDD5

AINN

AGND

ADS5400

AGND

Analog

Inputs

Bipolar

Transistor

Buffer

~0.3pF

Package

~0.2pF

Bondpad

~5.25nHBondWire

AVDD5

~0.3pF

Package

~0.2pF

Bondpad

Sampleand

Hold

1 Stage

OfPipeline

~5.25nHBondWire

2.5V

500 W

112 W

0.3pF

500 W

0.3pF

ADS5400

www.ti.com

SLAS611C – OCTOBER 2009 – REVISED JANUARY 2016

Feature Description (continued)

Figure 25. Analog Input Equivalent Circuit

For a full-scale differential input, each of the differential lines of the input signal swing symmetrically between 2.5

V + 0.5 V and 2.5 V – 0.5 V. This means that each input has a maximum signal swing of 1 VPP for a total

differential input signal swing of 2 VPP. The maximum fullscale range can be programmed from 1.5 to 2 VPP using

the SPI. The maximum swing is determined by the internal reference voltage generator and the fullscale range

set using the SPI, eliminating the need for any external circuitry for this purpose. The analog gain adjustment has

a resolution of 12-bits across the 1.5-2VPP range, providing for fine calibration of analog gain mismatches across

multiple ADS5400 signal chains, primarily for interleaving.

The ADS5400 obtains optimum performance when the analog inputs are driven differentially. The circuit in

Figure 26 shows one possible configuration using an RF transformer. Datasheet performance, especially at > 1-

GHz input frequency, can only be obtained with a carefully designed differential drive path to the ADC.

Figure 26. Converting a Single-Ended Input to a Differential Signal Using an RF Transformer

7.3.2 Voltage Reference

The 2 V voltage reference is provided internal to the ADS5400. A VCM (voltage common mode) pin is provided

as an output for use in DC-coupled applications, equal to the AVDD5 supply divided by 2. This provides the

analog input common mode voltage to a driving circuit so that the common mode is setup properly. Some

systems may prefer the use of an external voltage reference. This mode can be enabled by pulling the

ENEXTREF pin high. In this mode, an external reference can be driven onto the VREF pin, which is normally

expecting 2 V.

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