AD5535B driven capacity

We use the AD5535B evaluation board to provide 32 channel bias voltage for our silicon photon sensors(MPPC from Hamamatsu) that have a working voltage ranges from 65 to 70 volts with consumed current around 1 mA.
The AD5535B only provide a maximum source current around 550 uA. We tried to directly drive our photon sensor and it only goes up to about 64.4 volts.

I check the datasheet and found AD5535B provide much larger sink current, around -2 mA. My idea is to increase the driven capacity of the DAC output with a pull up resistor that connected to 100 V power source. Assuming the output voltage is 67 V, the input high voltage Vpp is 100 V, the pull up resistor with 17.5 k Ohms gives 2 mA current, then 1 mA goes to the photon sensor and the other 1 mA goes to the DAC as a sink current. I tested the output and found it only goes up to 67.2 V and can not goes up anymore. The datasheet only gives the Output Amplifier Source and Sink Capability with output voltage of 70 V, I want to know how it will change for different output voltage setting? Is there a detailed circuit diagram of the chip's output stage? Could you give some other suggestions to increase the chip's current driven capacity?