We are presenting comparative studies on two switched current (SI) differentiators; one is a classical version, the other is a cumulative version. Both are applied to smart pixel time-domain differentiation. In both differentiators, the differentiation is calculated by subtracting the instantaneous photocurrent from a previously sampled one. The key issue in this case is to obtain accuracy even with an extremely low current level (from pA to nA). The operability of both SI memories is verified through chip tests. Within an optical input dynamic range of five decades and a minimum level of 2 Lux (1pA in current), the maximum current reproduction error rate is 10% for the first one and 6% for the second. Furthermore, the cumulability of the second version is also confirmed by our tests.