National ADC12V170 manual ADC Reference and Input Common Mode, Board Outputs

ADC12V170 Evaluation Board User’s Guide

Figure 2. Analog Input Network of ADC12V170LFEB: FIN < 150 MHz

Figure 3. Analog Input Network of ADC12V170HFEB: FIN > 150 MHz

4.3 ADC Reference and Input Common Mode

The internal 1.0V reference on the ADC12V170 is used to acquire all of the results in the ADC12V170 datasheet. It is recommended to use the internal reference on the ADC12V170. However, if an external reference is required, the ADC12V170 is capable of accepting an external reference voltage between 0.9V and 1.1V (1.0V recommended). The input impedance

of the ADC12V170 VREF pin (pin 46) is 9 k￿. Therefore, to overdrive this pin, the output impedance

of the exernal reference source should be << 9 k￿.

It is recommended to use the voltage at the VRM pin (pin 45) of the ADC12V170 to provide the 1.5V common mode voltage required for the differential

analog inputs VIN+ and VIN-. The ADC12V170 evaluation board is factory-assembled with VRM

connected to the transformer center-tap through a

N

49.9￿ resistor to provide the necessary common mode voltage to the differential analog input.

4.4 Board Outputs

The digitized 12-bit output word from the ADC12V170 evaluation board is presented in interleaved double data rate (DDR) format. The digital output lines from the ADC12V170 evaluation board consist of 16 lines which are arranged into 8 LVDS pairs. These 8 pairs of lines carry the 12-bit output data (6 pairs), the DRDY signal which should be used to capture the output data (1 pair) and the over-range bit (OVR) which indicates that the digital output has exceeded the maximum digitizable signal (1 pair).

Since the data is presented in interleaved double data rate (DDR) format, the 12-bit word is output on 6 data pair lines with half of the data (odd bits: D1+/-, D3+/-,

- 5 -	www.national.com
	Rev 0.0