EVAL-AD7745/46EBZ Range Extension

Dear Community,

I'm using the EVAL-AD7745/46EBZ and I want to extend the capacitive input range as described in the AN-1585 to measure a capacitance up to 180pF.

I followed the description and setup but range extension doesn't work.

Following the AN-1585, I'm using a 100kΩ resistor for R1 and 120kΩ for R2.
The Cap Gain Calibration register reads a value of 0x63E0 resulting in F_Gain_Cal = 1.39 and C_DAC_EFF = 1.5782pF
C_sensor = 180pF leads to DAC_set = 114.054 -> 114 -> 0x72
Therefore the Cap DAC A register is set to 0xF2

Cap Setup register is 0x80 and EXC Setup is 0x2B
For conversion the Configuration register is set to 0x3A
The rest of the register values is set to default.

The setup still yields values for a capacitance up to 4pF but any higher input results in 0xFFFFFF for the conversion value.

First of all, how exactly does the range extension circuit for the EVAL-AD7745/46EBZ look like?
Figure 1 of the AN-1585 shows the circuit only for the AD7746 but not for the whole evaluation board on which the AD7746 is already connected to the host system.

Where do I get wrong here?

  • 0
    •  Analog Employees 
    on Oct 2, 2020 3:49 PM 1 month ago

    Please share the schematic of your setup.

    There is no range extension circuit on the Eval board. Are you using any external opamp between the EXC and CIN?

  • Yes, of course I'm using an external opamp.
    I'm using the AD8515 as descirbed in AN-1585.

    I'm using a Raspberry Pi  to read the data from the EVAL-AD7745/46EBZ via I2C.
    It works fine up to 4pF but range extension doesn't work as already described.

    My circuit looks like in the image file.
    Furthermore on the EVAL-AD7745/46EBZ, the 3.3V pin and the AVDD pin are connected on LK1.
    The EVAL-AD7745 (on J1) and the Raspi are both powered by their on power supply unit.

    Also, I have already disconnected the two 0 Ω SMD resistors on the EVAL-AD7745 to disconnect EXCA from the on-board length sensor demo. As described in the EVAL-AD7746EB data sheet on page 6.

    My problem remains still the same:

    Any capacitance input higher than 4pF results in 0xFFFFFF for the conversion value.

    How exactly does the range extension circuit for the EVAL-AD7745/46EBZ look like when used by an external microcontroller like the Raspi?

  • 0
    •  Analog Employees 
    on Oct 7, 2020 4:10 AM 1 month ago in reply to TKG

    Please share all the register settings that you have done.

    Have you probed both EXCA, EXCB, the junction of R1 and R2.

  • Cap Setup (address 0x07) set to value --> 0x80
    EXC Setup (0x09) --> 0x2B
    CAP_DAC_A (0x0B) --> 0xF2

    for offset calibration
    Configuration register (0x0A) --> 0x3D

    for single conversion
    Configuration register 0x0A --> 0x3A

    the other registers read their default value

    Have you probed both EXCA, EXCB, the junction of R1 and R2.

    What exactly do you mean?
    Do you refer with R1 and R2 to my circuit sketch or any data sheet?

    EXCA and EXCB are both working for external connection.