High Speed ADC Interface Analog Inputs Most of

High Speed ADC Interface

Analog Inputs Most of the HS ADCs have a differential Input • Two inputs signals, 180 degrees out of phase • ½ signal swing compared to Single Ended, which reduces the drivers distortion, thereby increasing system performance • Common mode noise rejection • Theoretically, differential signaling results in cancellation of evenorder harmonics • Typically gives best distortion performance, including high-IF In many applications the source to be measured is a kind of singleended, e. g. antenna (receiver), connector (oscilloscope)

ADC Inputs • • In a differential input ADC, the analog input waveforms should swing symmetric to the common mode voltage. Biasing Inputs – Analog inputs must be biased correctly. – Can be accomplished by using applying the VCM voltage to the center tap of a transformer or using a resistor bias. – Ex. ADS 5547 (14 bit, 210 MSPS) • Self-biased Inputs – ADC input buffer biases the input to the VCM. – AC couple the inputs using either a series capacitor or transformer. – Ex. ADS 5463 (12 bit, 500 MSPS) Bias? ? ? ADC

ADC Common Mode • ADCs have an internally generated common mode, however it can be set externally (VCM or VREF) • Internally generated common mode of the ADC can be used as an output to source the common mode of a driving amplifier – For ADS 5545 VCM output, it can source 4 m. A of current. • Common mode can be forced from an external common mode (ex. amplifier) – Follow the datasheet recommendation for range. External VCM range can vary widely between ADCs. Ex. ADS 5545 VCM =1. 5 V +/-100 m. V – Using external VCM can reduce the dynamic range and headroom.

Transformers • Transformers – – – – Isolated InputOutput – does not pass DC! Common Mode can be applied to the center tap to bias ADC inputs Can provide voltage step-up or step down Can provide single-ended to differential conversion Provides best AC performance, particularly at high-IF They are passive devices, no noise added Zp/Zs = (Np/Ns)2 = (Vp/Vs)2 Impedance Ratio = (Turns Ratio)2 = (Voltage Ratio)2 Used in HS ADC Characterization

Transformer Example Notice this ADC provides a pin called VCM. Not all do (even from TI).

Transformers (and baluns) • Limitations – – Phase Balance Amplitude balance Flatness of bandwidth Frequency Bandwidth

Transformers (cont. ) • Improved performance Dual transformer for better balance (minimize the mismatch of differential signal due to transformer winding capacitance) • Transformer is automatically AC coupled – Can use VCM to bias at center tap, or at termination • Balun is not AC coupled, need AC coupling external to balun

Low Frequency Analog Input Structure ADS 5545

ADC Input Circuit termination • Termination resistors are commonly used to bias the input signal to VCM • 50 ohm input from test equipment => 50 ohm termination after 1: 1 transformer • Larger termination resistors may lead to loss of SFDR – Due to common mode switching noise