OK, let’s try this again, your code:
writeData[0] = 0x1F; // What are we writing? 0x18 says A0, 0x1F is A7
spi.WriteRead(writeData, readData, 1); // Skip the first byte
You set writeData[0] which will tell the ADC which channel you want to read. The call to spi.WriteRead writes out the two byte array: 1F 00 and reads in a two byte array from the ADC. When you do that many times in a loop it writes out: 1F 00 1F 00 1F 00 1F 00 1F 00 1F 00 … (and for each pair you will read in two bytes)
Now take a larger write array, say 100 bytes and set every odd byte to 1F, now write out this whole array at one time, which would like: 1F 00 1F 00 1F 00 1F 00 1F 00 1F 00 … (hmm… looks suspiciously the same). So the ADC seeing the same exact data clock in should respond in exactly the same way (in other words your read array will contain 50 byte pairs of readings.
I need to place an electronics order tomorrow so I’ll grab on of the ADCs your using and give it a try.