为什么VSS的IR-Drop比VDD的大呢

SKILLER

我在block 设计时候，给了几个虚拟的P/G pair，然后给了1.8v的供电，但是得出来的结果是VSS的IR-Drop比VDD的大，这个应该作何解释呢？
下面是ICC报出来的一些report：
Total Current from Pad Cells:    0.00 mA (0.00%)
Total Current from Virtual Pads:  277.78 mA (100.00%)
Maximum IR drop in pre_pns : 74.15 mV
Maximum current in pre_pns : 75.334 mA
Maximum EM of wires in pre_pns : 5.398853e+01 A/cm, layer METAL2
Maximum EM of vias in pre_pns : 1.338229e+05 A/cm_square, layer VIA23
The PNS synthesizes the net VDD successfully
The maximum IR drop of the synthesized net VDD is  74.151 mV
Total Current from Pad Cells:    0.00 mA (0.00%)
Total Current from Virtual Pads:  277.78 mA (100.00%)
Maximum IR drop in pre_pns : 245.09 mV
Maximum current in pre_pns : 126.914 mA
Maximum EM of wires in pre_pns : 9.712683e+01 A/cm, layer METAL2
Maximum EM of vias in pre_pns : 3.097430e+05 A/cm_square, layer VIA23
Warning: Target IR drop for net VSS cannot be satisfied. (PNA-108)
The maximum IR drop of the synthesized net VSS is 245.091

SKILLER

自己先顶一个吧，report中上面的是VDD的分析，下面的是VSS的分析，期待大婶的精彩解答！！！

cxj2010

回复 1# SKILLER


   请问你是用哪个命令或者哪个选项得到这个报告的，我的怎么都没有 呢？

芝芝莓莓

请问楼主找到原因了吗？