Connecting AD8436 output buffer to Differential Amplifier


I am trying to connect the output buffer which is part of the AD8436 RMS-to-DC device to a differential amplifier, namely the TI THS4521.

The output buffer is configured as stated in Fig. 35 of the AD8436 datasheet ( see Output_Buffer.png) .

The  differential amplifier THS4521 is configured as shown in the attachment ( see Diff_Amp.png ) omitting the Resistors R_IT since I am assuming that the output impedance of the buffer inside the AD8436 is very low.

The problem is that when I connect the output buffer directly to the differential amplifier its output goes up to a high constant value.

When I introduce an additional voltage follower (AD8512) between the output buffer and the differential amplifier all works fine.

So,  what am I missing? My interpretation of this observation is, that the output impedance of the output buffer inside the AD8436 must be very high.  But isn't it the point of this buffer to provide a low output impedance?

Also since I am very fond of ADI products is there an functional equivalent of the THS4521 in the ADI portfolio ?

Thank you very much for any advice!

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