What aspect of receiver analog-to-digital conversion determines the maximum receive bandwidth of a direct-sampling software defined radio (SDR)?
Why In a direct-sampling SDR the RF is digitized straight away, so the Nyquist criterion applies: the highest frequency that can be represented without aliasing is half the sampling rate. Double the sample rate and you double the usable receive bandwidth. The sample rate is therefore the parameter that sets maximum bandwidth, while other converter specs affect signal quality rather than span.
Watch out Sample width in bits is the tempting pick, but the number of bits sets dynamic range and quantization noise floor (roughly 6 dB per bit), not how wide a slice of spectrum you can capture. Integral and differential non-linearity are accuracy errors in the converter's transfer function that create spurious products.
Rate = width of spectrum, bits = depth of dynamic range. Bandwidth stops at half the sample rate (Nyquist).