I am using the Ad630 as a lock-in amplifier. The problem I have with the chip is that when I have no signal coming in, a 250mV peak-peak square wave signal at the reference frequency appears on the output of the AD630. When there is signal coming in, the signal rides on top of this square wave. How to get rid of this square wave or reduce it to the uV to nV range? The chip is configured the same way it is shown on the data sheet. Pins 1 and 16 have the signal coming in from a trans impedance circuit. Pins 15,19 and 20 shorted together and pins 17 and 14 grounded. I use pin 10 for reference input and ground pin 9 or vice versa. The reference signal is pulsed at 1kHz from -2.5V to 2.5V. The output of AD630 is fed into a low pass filter.