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# How to decide load resistor for best performance at DAC voltage output?

Category: Datasheet/Specs

hello,

I tried to design RS170(voltage output level 1V) application board with ADV7391 using full-drive (Rset: 510ohm, RL: 37.5 ohm).

In ADV7391 data sheet rev.j, the recommended load resistor consists of 37.5 ohm to expect 34.7mA full-scale current(1.3V) at p.72, p.69, and p.7.

However, fig. 93 shows that the recommended RL is 75 ohm. At this point, I'm curious about which value is best for the performance.

At summary, my questions are

1. If only 75ohm load resistor satisfies the best performance @ fig.93, what is the relation between Rset and RL with Vaa to calculate DAC voltage output level?

2. In ADV7179 datasheet, they describe load resistors 75ohm for the best performance. If the fact also can adapt to the ADV7391, is the based theory doubly terminated 75ohm load as described in AN205 application note and CN-0264(rev.0)?

3. If only load resistor 75 ohm is adopted without monitor impedance , which level of the output current is right? (34.7mA / 2 or 34.7mA x 2)

Best regards,

mcpark

[edited by: mcpark09 at 4:14 AM (GMT -5) on 29 Dec 2023]
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• ADI North America will be on winter shutdown starting December 25, 2023; perhaps another community member can assist you until our return on January 8, 2024.
• Hi,

Full drive works fine if you are only driving one load (75 Ohm).  If you are driving 4 loads without buffers, then the DAC output will see 75 || 75/4 = 15 Ohm load. The DAC output at full load is designed for 37.5 Ohm. So, you will need to buffer the output.

When Rset = 510 Ohms, the DAC outputs expect to see a 37.5 Ohm load. The load consists of a 75 Ohm resistor to ground right next to the pin and a 75 Ohm resistor to ground at the sink. Normal cables plus sinks are set up this way.

The encoders expect to be terminated with 75 ohms on your PCB with another 75 ohms at the end of the transmission line, so the total load resistance is 37.5 Ohms.
When configured this way the output will be 1Vpp.

As stated, DAC output current is proportional to Rset and Rload. Full drive mode is defined as 34.7mA with 37.5 Ohm load and 0xFF digital inputs giving an output of 1.3V. Using the +7.5% gain would be 1.4.

Thanks,
Dharani S

• Hi,

Full drive works fine if you are only driving one load (75 Ohm).  If you are driving 4 loads without buffers, then the DAC output will see 75 || 75/4 = 15 Ohm load. The DAC output at full load is designed for 37.5 Ohm. So, you will need to buffer the output.

When Rset = 510 Ohms, the DAC outputs expect to see a 37.5 Ohm load. The load consists of a 75 Ohm resistor to ground right next to the pin and a 75 Ohm resistor to ground at the sink. Normal cables plus sinks are set up this way.

The encoders expect to be terminated with 75 ohms on your PCB with another 75 ohms at the end of the transmission line, so the total load resistance is 37.5 Ohms.
When configured this way the output will be 1Vpp.

As stated, DAC output current is proportional to Rset and Rload. Full drive mode is defined as 34.7mA with 37.5 Ohm load and 0xFF digital inputs giving an output of 1.3V. Using the +7.5% gain would be 1.4.

Thanks,
Dharani S

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