Is ab under 498a a bar to joining govt service

Yes, Anticipatory Bail is immunity from Detention and Custody and Not arrest. Hence AB becomes effective only on arrest, and reduces a non bailable offence to bailable. Hence technically it is arrest, and is required to be disclosed to employer. 

The attestation form required to be submitted to the Govt is havign specific column to report arreste, bal andpending criminal case and such form goes to the Police station where you resided in the last five years. 

Is acquittal also required to be informed to employer?

The colums of attestation form include all previous criminal record including acquittal to be reported. 

If already in service even bail is to be reported. So without reporting acquittal one is not clear from vigilance angle to earn promotion or pension.