We present the design and characterization of a CMOS pixel direct charge sensor, Topmetal-IIa, fabricated in a standard 0.35µm CMOS process. The sensor features a 45 × 216 pixel array with a 40µm pixel pitch which collects and measures external charge directly through exposed metal electrodes in the topmost metal layer. Each pixel contains a low-noise charge-sensitive preamplifier to establish the analog signal, which is read out through time-shared multiplexing over the entire array. Compared to the earlier Topmetal-II- chip, the analog readout noise of Topmetal-IIa is reduced by 10.8% from 13.9e- to 12.4e-, and the DC voltage variation noise is reduced by 21% from 1.2mV down to 0.946mV. The sensor is capable of detecting both electrons and ions drifting in gas. These characteristics enable its use as the charge readout device in future Time Projection Chambers without gaseous gain mechanism, which has unique advantages in low background and low rate-density experiments.