  The ARGO-YBJ experiment has been in stable data taking from November 2007
till February 2013 at the YangBaJing Cosmic Ray Observatory (4300 m a.s.l.).
The detector consists of a single layer of Resistive Plate Chambers (RPCs) (
about 6700 m^2}) operated in streamer mode. The signal pick-up is obtained by
means of strips facing one side of the gas volume. The digital readout of the
signals, while allows a high space-time resolution in the shower front
reconstruction, limits the measurable energy to a few hundred TeV. In order to
fully investigate the 1-10 PeV region, an analog readout has been implemented
by instrumenting each RPC with two large size electrodes facing the other side
of the gas volume. Since December 2009 the RPC charge readout has been in
operation on the entire central carpet (about 5800 m^2). In this configuration
the detector is able to measure the particle density at the core position where
it ranges from tens to many thousands of particles per m^2. Thus ARGO-YBJ
provides a highly detailed image of the charge component at the core of air
showers. In this paper we describe the analog readout of RPCs in ARGO-YBJ and
discuss both the performance of the system and the physical impact on the EAS
measurements.
