AD7746_CONF_VTFS_MASK
idx = FIELD_GET(AD7746_CONF_VTFS_MASK, chip->config);
chip->config &= ~AD7746_CONF_VTFS_MASK;
chip->config |= FIELD_PREP(AD7746_CONF_VTFS_MASK, i);
idx = FIELD_GET(AD7746_CONF_VTFS_MASK, chip->config);