AuthLDAP plugin should read LDAP parameters from config/ldap.php or they both should have central config db. Ldap.php has more options like filters and groups.

It would be nice to have also optional Kerberos support. Basically it means that before showing login form you check for logged in username given by your webserver: isset($_SERVER['REMOTE_USER']) and if set, you log it in. There is no need for password in that case, as authentication is made by Apache/IIS/nginx.
You can still make a LDAP query to get memberOf values (groups) for the user and check if the user is allowed to log in (LDAP group should be defined in settings by administator, example: CN=LimeAdmin,OU=groups,DC=company,DC=com).

I made the same functionality for Piwik, you can get ideas from:
https://github.com/tehnotronic/PiwikLdap/blob/master/LdapFunctions.php
and https://github.com/tehnotronic/PiwikLdap

Sadly I am not aware with limesurvey architecture to complete this request myself but I am glad to test and help finding bugs.