A novel approach to perform photonic analog-to-digital conversion with improved bit resolution is proposed and investigated. Instead of using Mach-Zehnder modulators (MZMs) with geometrically-scaled half-wave voltages, the MZMs in the approach have identical half-wave voltages, which greatly simplifies the implementation. To improve the bit resolution without increasing the number of MZMs, each MZM is connected with multiple comparators having multiple thresholds. The quantization and encoding are performed based on the symmetrical number system (SNS) technique. Three new quantization and encoding schemes based on the SNS are proposed and demonstrated. A 4 bit photonic analog-to-digital conversion based on the given schemes is investigated. For the given schemes, two MZMs are needed with the numbers of comparators being 14, 16, and 9, respectively. Numerical simulations and experiments are performed. The effectiveness of the proposed schemes is verified.