Haifeng Wu

March 24, 1999

The 3in1 card uses +5v, -5v, and +15v power supplies. For each of these input voltages, there is a 1uF decoupling capacitor and a power supply filter on every card.

In order to investigate the power supply noise rejection ability of the 3in1 card, we added artificial sinusoidal noise of the desired frequency to the power supply. Two transformers were used to produce low frequency or high frequency noise. The transformers were placed in series with the power leads from the each of the supply voltages in turn. The primary winding of the low frequency transformer had about 80 turns, while the secondary had 8. The two windings of the high frequency transformer had 8 turns and 1 turn respectively. The magnetic material of the transformers was the same kind of ferrite.

The low frequency transformer was capable of producing noise between 50Hz and 10KHz, and the high frequency transformer to 100KHz. The primaries of the transformers were driven by a high gain voltage source. The two leads of the transformer secondary were connected between the power supply and corresponding 3in1 card power input.

Noise was generated on only one supply voltage at a time. The sine wave was generated by a programmable function generator AWG2040, and fed into the input of a spare 3in1 card, the output from which drove the transformer.

The setup of the test is shown in the figure on the next page.

Noise rejection ratio or NRR is defined as

NRR = 20log ( Vni/Vno)

Here Vni is the power input noise measured at the power input terminal of 3in1 card, and Vno is signal output noise measured at differential output of 3in1 card. In order to be accurate, the FFT function of the digital scope was used to measure the amplitude of specific noise frequency.

All three different voltage power were tested at 50Hz, 100Hz, 1KHz, 10KHz, 100KHz. The result is shown bellow.

POWER SUPPLY NOISE REJECTION TEST RESULT

-5v power supply

+5v power supply

+15v power supply

For both high gain and low gain, it is desired that the output noise of 3in1 card caused by power supply noise should less than +- 0.5 ADC counts. Thus the perspective power supplies to the 3in1 card must meet the noise specification listed at the following table, in order to make sure it wouldn't cause more than 0.5 ADC counts in the 3in1 card output of both gain channel.