Post Go back to editing

# AD5535B using all channels together

Hi, I need an ADC for 200V but only one output.

On the AD5535B each channel can drive: 550 µA, I wonder if is possible to use all together to supply more current? 0.55uA*32 = 17.6mA. I know that 550uA is the maximum, not recommended for continuous use. Actually, my power supply has 8mA max.

The output buffer amplifier is specified to drive a load of 1 MΩ and 200 pF

So 200V/1M = 200uA is the expected usage.

Is it possible to shortcircuit outputs to get more output current? 32*.2mA = 6.4mA?

Fig 8 shows that even with a load between outputs the short circuit protection turns off the output. But maybe because 200v/10k = 20mA and outputs can behave like sinks of 2.1mA which is more than the 550uA source.

Would it be possible to override the protection using a fast diode on each output? Forcing the outputs to behave like sources and preventing them to sink?

2. Max output current at 200V

From the specs table, I understand that with VPP = 215V and setting an output of 200V each output can source 550uA is it correct? Fig 10 doesn't mean that only for 70V we can get 550uA right? With an output of 200V, the maximum current is 550uA, ok?

3. Setting all the outputs at the same time

Setting all the outputs at the same time is not possible right?

A4 to A0 Bits The A4 to A0 bits can address any one of the 32 channels. A4 is the MSB of the address, while A0 is the LSB.

There's no register for "all"

SCLK works up to 30Mhz, each set requires 19b but really becomes 3x8 packets, we could write each channel on approx 1us, ok? So for 32 channels = 32us

The user must allow 200 ns (minimum) between successive writes.

If we cannot set all at the same time since there's no latch option, when the first channel set from 0 to 200v will try to source all the current to the load and It'll protect himself going to 0v right? Will it recover once the other channels start to provide current to the load?

4. Price

Why is the evaluation Board cheaper than the chip alone?

Thanks!!!

• Hi,

May I know what your application is?

Kindly see below for my response regarding your concerns:

1. The 0.55mA short circuit specification is actually the current when you try to short the output. The minimum resistive load is 1Mohm, so you are right about 200uA as the maximum operating load current at 200V output.

It is a bit tricky to short the outputs to achieve a higher load current capability. You have to make sure that the differences between the outputs are very small and that they turn on to the same value at the same time. With this, you would draw a larger current from the higher voltage outputs which could easily exceed the maximum operating load current.

Adding diodes would also add to the offsets between the output as they don't have exact forward bias voltages.

2. As I mentioned on item 1, the short circuit current is the current (550uA) is the current at the output when you try to short it. Looking at Figure 10, the output will start to drop when the current exceeds ~800uA. The AD5535B outputs are recommended to have high impedance loads (>=1Mohm).

3. You can only write to a single DAC at a time, when shorted to another output which is at 0V, the output will start to fall. The AD5535B doesn't have a short circuit protection or thermal shutdown capability so the part may got damaged when this event happens.

4. We need to review this and see if there should be any adjustments to be made. But as of the moment, the pricing stays as it is.

Best regards,

Rainier

• Thanks for your answers. I'll look for another option for 200V.