We propose a capability to record level histogram of digitized signals for the sake of accurate amplitude calibration. Current ALMA correlators apply Van Vleck correction based on autocorrelation amplitude under assumption that quantization voltages in the digitizers are equally spaced. The real ALMA digitizers have 3-4% non-uniformity in 8 quantization voltage levels (CSV-2311). The non-uniformity can cause a systematic error of 9.6% at worst case or 2.1% optimized case in autocorrelation amplitude. By recording histogram of quantized level, we can estimate the quantization thresholds and then apply better accurate Van Vleck correction. The level histogram is also useful for digital power measurement instead of autocorrelation.