How to add content at the end of posts?

I am trying to add custom fields at the end of posts.

I succeeded showing the custom content using the below mentioned code. But the main post’s content was replaced. :

function custom_content_filter_the_content( ) {
   the_field("field1");
}
add_filter( 'the_content', 'custom_content_filter_the_content' );

So I want to try this as in below mentioned example:

function custom_content_filter_the_content( $content ) {
    $custom_content = 'some_content_appears_here';
    $custom_content .= $content;
    return $custom_content;
}
add_filter( 'the_content', 'custom_content_filter_the_content' );

Want to replace 'some_content_appears_here' with <span class="custom_fds"><?php if( function_exists('the_field') ) the_field('field1'); ?></span>.

Could any tell me how to use the <?php the_field() ?> with in functions.php to add custom fields(created with Advanced custom fields) at the end of the post content(that is easy when we use in templates but not in funcitons.php) ?

Update:
@G.M ‘s code helps me to achieve what I want.

Here is the final code:

function custom_content_filter_the_content( $content ) {
    if ( function_exists('get_field') ) {
      $content .= '<span class="custom_fds">' . get_field('field1') . '</span>';
      $content .= '<span class="custom_fds">' . get_field('field2') . '</span>';          
    }
    return $content;
}
add_filter( 'the_content', 'custom_content_filter_the_content' );

Thanks to @G.M. I have a few questions related to the custom content.

• I am displaying multiple fields in the single function. Is that ok?
• Instead of the end or beginning of the post, is there a way to choose a position for custom content any place with in the_content ? For example, if I use any plugins for social sharing, they display the content at the same place, I want to change positions of these two types of custom content.