AMC7823: Help review the schematic design and give some design suggestions?

Hi Xiaofu, 

Erin is reviewing the schematic and will get back to you within a few days. 

Best,

Katlynne Jones

Hi Xiaoful,

Thank you for your patience. 

For the CH6 input: You would want to lower the impedance on the input of the channel. Current is needed to charge the sampling capacitor for the SAR ADC - I would suggest lowering the resistor divider to 10k/1k instead of 100k/10k, or adding a buffer if you would prefer to not use current from the 28V voltage source.

Other than that, I don't believe you'll have any issues.

Thank you,
Erin

Hi  Guthrie:

Thank you very much for your reply. I consulted Xiao Fu about this question. Let me describe it in detail:

1. In fact, we do not use the ADC of CH6. Even if there is a problem here, will it affect other channels?

2. The problem is that when the IC is actually used, there will be a probability of anomalies. In this case, the reg of all dac channels is normal, and link problems are eliminated. However, all ADC channels will sample full 00 or full FF. In fact, the voltage of the dac is not really output at this time. After manually writing the reset reg of 7823, the 7823 chip will return to normal.

3. Each time we reboot the device, the entire RF board is powered off. After the device is rebooted, the 7823 soft-reset reg is performed. I now add a logic, reboot after the first operation 7823 reset pin, and then soft reset 7823.

thanks

tianyi

Hi Wu,

1) All channels will need a sufficient amount of current to properly measure voltage. If only CH 6 has a problem, though, it will not affect other channels. 

Doing a full reset of the device upon startup is a good way to avoid problems with the device, and you shouldn't have any issues with that.

Thanks,
Erin

Hi Guthrie：

   Thank you very much for your reply.I would like to ask if other customers or your internal testing encountered similar problems? This looks more like a chip problem, or no initialization at all.

Thanks

Tianyi

Hi Wu,

Is the device being powered up in the correct sequence? Here's the order from the datasheet:

1) AVDD
2) DVDD
3) BVDD
4) External Reference (if used)

SS Pin is recommended to be pulled up to BVDD during start up (It looks like this is DNI in your schematic)
ELDAC Pin should be grounded during start up.
In general, digital pins should be grounded during start up.

Datasheet Page 37:
"After power-on or reset, all registers are reset to the default values (see Table 2). In order for the device to work properly, BVDD must not be applied before DVDD and AVDD are applied, and DVDD must not be applied before AVDD is applied. All three supplies must power up before the external reference voltage (if any) is applied. Additionally, because the DAC input shift register is not reset during a power-on reset (or during a hardware reset or software reset), the SS pin must not be unintentionally asserted during power-up of the device. It is recommended that the SS pin be connected to BVDD through a pull-up resistor to avoid improper power-up.

Likewise, the state of the ELDAC pin must be maintained at ground potential during power-up. To ensure that the ESD protection circuitry of this device is not activated, all other digital pins must remain at ground potential until BVDD is applied."

Thanks,
Erin