How to calculate the point where AD8370 has the same gain in both low gain mode and high gain mode. I need to switch smoothly between the two modes (for AGC (automatic gain control) construction). In the datasheet there is only a formula for calculating gain in high gain mode.
The formula on p. 15 of the datasheet describes the gain difference being 17 dB (i.e., ~ 7.08 X) when flipped between the low and high gain ranges (MSB= 0 or 1). I low gain mode, PreGain = 1, and in high gain mode, PreGain = 7.08 V/V.
The theoretical linear voltage gain can be expressed with respect to the gain code as:
AV = GainCode Vernier (1 + (PreGain − 1) MSB) where:
AV is the linear voltage gain.
GainCode is the digital gain control word minus the MSB(the final 7 bits).
Vernier = 0.055744 V/V
PreGain = 7.079458 V/V
MSB is the most significant bit of the 8-bit gain control word. The MSB sets the device in either high gain mode (MSB = 1)or low gain mode (MSB = 0).
For example, a gain control word of HG45 (or 10101101 binary) results in a theoretical linear voltage gain of 17.76 V/V,calculated as:
45 × 0.055744 × (1 + (7.079458 − 1) × 1)
The correct equations are :
High gain mode:
Av = GainCode x Vernier x (1 + (PreGain-1) x MSB)
Low gain mode: (PreGain = 0)
Av = GainCode x Vernier x (1 + (1-1) x MSB)