How do I sign into a Google app on my iPhone without signing in for ALL apps?

I like to use the Google Voice iOS app on my iPhone to make and receive calls. However, if I sign into my Google account in the Google Voice app, suddenly all the other Google apps (such as Google Maps) also know who I am. I don't like this, because Google Maps has no business of knowing which Google account is using Google Voice on the same phone. How can I avoid this?