OVR Detection Threshold

-25

-20

-15

-10

-5

0

0

2

4

6

8

10

12

14

16

Programmed Value (1-15)

500Ω

500Ω

Vcm= 1.9V

INA_P

INA_N

2nH

0.5Ω

1.3pF

20Ω

1.4pF

2nH

0.5Ω

1.3pF

20Ω

1.4pF

ADS5402

www.ti.com

SLAS936 – MARCH 2013

ANALOG INPUTS

The ADS5402 analog signal inputs are designed to be driven differentially. The analog input pins have internal

analog buffers that drive the sampling circuit. As a result of the analog buffer, the input pins present a high

impedance input across a very wide frequency range to the external driving source which enables great flexibility

in the external analog filter design as well as excellent 50

Ω matching for RF applications. The buffer also helps to

isolate the external driving circuit from the internal switching currents of the sampling circuit which results in a

more constant SFDR performance across input frequencies.

The common-mode voltage of the signal inputs is internally biased to 1.9V using 500

Ω resistors which allows for

AC coupling of the input drive network. Each input pin (INP, INM) must swing symmetrically between (VCM +

0.25V) and (VCM – 0.25V), resulting in a 1.0Vpp (default) differential input swing. The input sampling circuit has

a 3dB bandwidth that extends up to 1.2GHz.

OVER-RANGE INDICATION

The ADS5402 provides a fast over-range indication on the OVRA/B pins. The fast OVR is triggered if the input

voltage exceeds the programmable overrange threshold and it gets presented after just 12 clock cycles enabling

a quicker reaction to an overrange event. The OVR threshold can be configured using SPI register writes.

The input voltage level at which the overload is detected is referred to as the threshold and is programmable

using the Over-range threshold bits. The threshold at which fast OVR is triggered is (full-scale × [the decimal

value of the FAST OVR THRESH bits] /16). After reset, the default value of the over-range threshold is set to 15

(decimal) which corresponds to a threshold of 0.56dB below full scale (20*log(15/16)).

Copyright © 2013, Texas Instruments Incorporated

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